NCERT 6TH CLASS MATHEMATICS CHAPTER KNOWING OUR NUMBERS
PREDECESSORSUCCESSOR
Predecessor is 1 less than the givOF A NUMBERen number. For Ex. : Predecessor of 59028 is 59027.
Successor is 1 more than the given number. Successor of 9999 is 10000.
COMPARISON OF NUMBERS:
(a) Greater number has more number of digits.
(b) To compare two numbers having same number of digits, start comparing from the leftmost position.
If the leftmost digits are equal, move to the next digits.
Ex. Which is greater of 270346 and 48356?
Sol. 270346 has 6 digits
48356 has 5 digits
6 digits are more than 5 digits
270346 is greater than 48356
Greater number has more number of digits.
Ex. Find the greatest and the smallest numbers from the following group of numbers :
23787, 6895, 24569, 24659
Sol. Greatest number : 24659
Smallest number : 6895
ASCENDING AND DESCENDING ORDER
Ascending order 
arrangement of numbers from smallest to largest
Descending order 
arrangement of numbers from largest to smallest
Ex. Arrange the following number in ascending order :
257536, 38952, 385081, 365062
Sol. The smallest number is 38952. Other numbers greater than 38952, in order are 257536, 365062 and 385081.
The numbers in ascending order are :
38952, 257536, 365062, 385081
Ex. Arrange the following numbers in descending order :
19710, 887151, 453212, 925473
Sol. The greatest number is 925473. Other numbers smaller than 925473 in order are 887151, 453212 and 19710.
The numbers in descending order are :
925473, 887151, 453212, 19710
USE OF COMMAS
While reading and writing large numbers, it creates confusion as there are many digits in a number. To avoid making mistakes,
we use commas to divide the number into different periods. We can divide a number into different periods by using either the lndian
system of numeration or the lnternational system of numeration. But first let us learn about place value.
PLACE VALUE
The basis of the number system is place value. It is this place value which gives value to the number.
Definition :
Face value of a digit in a numeral is the value of the digit itself at whatever place it may be.
Place value of a digit in a given number is the value of the digit because of the place or the position of the digit in the number.
Placevalue of a digit = Facevalue of the digit × value of the place
Place value and Face Value :
Every digit has two values —
the place value and the face value. The face value of a digit does not change while its place value changes according to its position and number.
Number 
Digit 
Face Value 
Place Value 
53,694 
5 
5 
50,000 
3 
3 
3,000 

6 
6 
600 

9 
9 
90 

4 
4 
4 
Expanded form of a Number :
If we express a given number as the sum of its place value, it is called its expanded form.
Ex. Express
(i) 3,64,029
(ii) 2,75,00,386 in expanded form.
Sol. Place value of 3 = 3 ×
Place value of 4 = 4 × 1000
Place value of 0 = 0 × 100
Place value of 2 = 2 × 10
Place value of 9 = 9 × 1
The expanded from of 3,64,029 is 3 × 100000 + 6 × 10000 + 4 × 1000 + 0 × 100 + 2 × 100 + 9 × 1
INTERNATIONAL SYSTEM OF NUMBERS
Billions Milllions Thousands
Hundred Billion 
Ten Billion 
One Billion 
Hundred Million 
Ten Million 
One Million 
Hundred Thousad 
Ten Thousand 
One Thousand 
100, 000,000, 000 12 Digits 
10,000,000,000 11Digits 
1,000,000,000 10 Digits 
100,000,000 9 Digits 
10,000,000 8 Digits 
1,000,000 7 Digit 
100,000 6 Digits 
1 0,000 5 Digits 
1,000 4 Digits 
Reading and Writing of Numbers :
In lndian System of numbers, we divide the given number into periods starting from the right.
The first period called units period consists of 3 digits while each of the next periods called thousands
period, lakhs period, crores period respectively consists of 2 digits.
Each period is separated by a comma (,).
ln lnternational System of Numbers we make groups of 3 digits starting from right and separate each group by using a comma (,).
Now consider the six digit number 764325. In the lndian system, the number 7 is in the place of lakh and therefore the number is
named as seven lakh sixtyfour thousand three hundred twentyfive. lt is written as 7,64,325. In the lnternational system,
this numebr is named as seven hundred sixtyfour thousand three hundred twentyfive and is written as 764,325.
he first three digits form the righthand side are considered as ones, i.e., 325 ones and the next three digits to the
left of the digit 3 are considered as thousands, i.e. 764 thousands.
Ex. Write the name of the numbers according to lndian system of numeration.
(a) 1275834 (b) 30870209
Sol. (a) 12,75,834 = Twelve lakh seventy five thousand eight hundred thirty four
(b) 3,08,70,209 = Three crore eight lakh seventy thousand two hundred nine
Ex. Write the name of the number according to lnternational system of numeration.
(a) 7452283 (b) 48049831 (c) 699985102
Sol. (a) 7452283 = 7,452,283 = Seven million four hundred fifty two thousand two hundred thirty three.
(b) 48049831 = 48,049,831 = Forty eight million forty nine thousand eight hundred thirty one.
(c) 699985102 = 699,985,102 = Six hundred ninety nine million nine hundred eighty five thousand one
hundred two.
MAKING NUMBERS
(a) Making number without repetition of digits :
In case of nonrepetition of digits, it is better if we start making the number from left.
Ex. Write the greatest and the smallest 5digit numbers by using each of digits 8, 4, 7, 0, 2 only one.
Sol. For the greatest number, we write the greatest digit 8 in the Tthousands column. Next smaller digit in the thousands column and so on.
The greatest number = 87420.
T  T h 
T h 
H 
T 
O 
8 
7 
4 
2 
0 
For the smallest number, we write the smallest digit in the Tthousands column. But here 0 is the smallest digit. 0 is not written
on the extreme left of a number. So, we write 2 in the Tthousands column and 0 in the thousands column, Next digit greater than
2 is written in the hundreds column and so on.
The smallest number = 20478
T  T h 
T h 
H 
T 
O 
2 
0 
4 
7 
8 
Ex. Make the greatest and the smallest 5digit numbers using any five different digit with 4 in the tens place.
Sol. First of all write 4 in the tens column. For the greatest number, we write the greatest digit 9 in the
T  thousands column, next smaller digit in the thousands column so on.
T Th 
Th 
H 
T 
O 
9 
8 
7 
4 
6 
The greatest number = 98746
For the smallest number also, write 4 in the tens column first of all. Then write 0 in the thousands column.
Write 1 in the Tthousands column. Next greater digit in the hundreds column and so on.
The smallest number = 10243
T Th 
Th 
H 
T 
O 
1 
0 
2 
4 
3 
(b) Making number with repetition of digits : ln case of repetition of digit, it is better if we start making number from right.
Ex. Write the greatest and smallest numbers of 4 digits using all the digits 8,0,5.
Sol. For greatest number, select the smallest digit 0 and write in the ones column, Next greater digit is written in the tens column.
Next greater digits 8 is written in the hundreds column. Since no digit greater than 8 given, so we repeat 8 in the thousands column.
T Th 
H 
T 

O 
8 
8 
5 

3 
Greatest Number
The greatest number = 8850
For smallest number, select the greatest digit 8 and write in the ones column.
Next smaller digit in tens column and so on. Repeat the smallest digit in the end.
But here 5 is smaller than 8 and then comes 0 which cannot be repeated in the end.
So, we write 5 in the end and repeat 0 in the tens place.
Th 
H 
T 
O 
5 
0 
0 
8 
Smallest Number
The smallest number = 5008
NOTE :
i. For greatest number, greatest digit is repeated.
ii. For smallest number, smallest digit is repeated.
ESTIMATION IN NUMBER OPERATIONS
You must have come across news headlines involving large numbers. For example, ‘50,000 people participated
in the marathon.’ ‘5 lakh people gathered for a dip in the Ganges.’
We also see and read news about disasters, strikes, bandhas, etc.
For example, ‘80 crore rupees lost due to the fire.’
‘9 lakh people evacuated.’
‘The loss due to bandh is 1 crore.’
The numbers mentioned above do not represent the exact number of people or rupees.
They are only speaking of a nearest value.
Rounding up off numbers is an approximation. This means that when we speak of 50,000 people,
what we really mean is the crowd was between 45,000 and 55,000.
This rounding up helps us to get aproximate answers in addition, subtraction,
multiplication and division. Suppose Rs.3847 and Rs.8348 are the costs of a DVD player and a T.V., respectively.
If a friend asked you how much you paid for these two items, you need not to add the correct value of each.
You can say that it costs you around Rs.12,000. When we are cooking for a party, the approximate number of people
expected for the party, and the approximate quantity of each item required per person are considered. Similarly, the quantity
of things to be purchased for a party, the amount of cement to be purchased to construct a wall, the amount of paint to be bought
to paint a house are all approximations.
Estimating (Rounding) to the Nearest Ten :
To round off a number to the nearest ten consider the ones digit.
If the ones digit is 5 or greater than 5, then change the ten’s digit to the next higher digit and ones digit to zero.
If the ones digit is less than 5, then leave the tens digit unchanged but change the ones digit to zero.
Estimating (Rounding) to the Nearest Hundred :
To round off a number to the nearest to the nearest hundred,
consider the tens digit. If the ten’s digit is 5 or greater than 5, then change the hundreds digit to the next higher digit
and tens, ones digits to zeros. If the tens digit is less than 5, then leave the hundreds digit unchanged but change the tens and ones digits to zeros.
Estimating (Rounding) to the nearest Thousand :
To round off a number to the nearest thousand, consider the hundreds digit.
If this digit is 5 or greater than 5, then change the thousands digit to the next higher digit are change all the other digits before that to zeros.
If the hundreds digit is less than 5, then leave the thousands digit unchanged but change all the other digits before that to zero.
Estimation in Sums or Differencess :
Ex. Estimate and compare with the actual sum
(a) 730 + 998 (b) 12904 + 2888
Sol. (a) We round off to the nearest hundred
730 is rounded off to 700
998 is rounded off to 1000
Estimated sum = 1700
700+1000=1700
Actual sum = 730 + 998 = 1728
730+998=1728
Estimation is quite reasonable,
(b) We round off to the nearest tousand 13000
12904 is rounded off to 13000
2888 is rounded off to 3000
Estimation is quite reasonble
Actual sum = 12904 + 2888 = 15792
Estimation is quite reasonable
12904+2888=15792
Ex. Give a rough estimate and also a close estimate of 439 + 334 + 4317
Sol. Rough estimate : 400 + 300 + 4000 = 4700
For closer estimate, we round off to the nearest hundred
439 is rounded off to 400 400
334 is rounded off to 300
4317 is rounded off to 4300
Closer estimated sum = 5000
Ex. Estimate 8325 – 491
Sol. If we round off to thousand, we get
8325 rounds off to 8000
491 rounded off to 0
Estimated difference = 8000 – 0 = 8000
This does not give a reasonable difference, so we round off to the nearest hundred.
8325 rounds to 8300
491 round to 500
Estimated difference = 7800
This is reasonable estimation.
8300–500=7800
Estimation in Products : While estimating in products, we adopt the following rules :
(i) Round each factor to its greatest place, in other words, if a factor contains 2 digits,
round it off to the nearest ten and if it contains 3 digits, then round it off to the nearest hundred and so on.
(ii) Do not round off any 1digit factor.
Ex. Estimate the following products :
(a) 87 × 313 (b) 9 × 795 (c) 898 × 785
Sol. (a) 87 is rounded off to 90
313 is rounded off to 300
Estimated product = 90 × 300 = 27000
(b) 9 is not rounded off [ it is a onedigit no.]
795 is rounded off to 800
Estimated product = 9 × 800 = 7200 (c) 898 is rounded off to 900
785 is rounded off to 800
Estimated product = 900 × 800 = 720000
Estimation in Quotients :
In the process of estimation in quotients, we round off the divided and the divisor before the process of division.
Ex. Estimate the following quotients :
(a) 81 ÷ 17 (b) 7525 ÷ 365
Sol. (a) 81 is rounded to 80
17 is rounded to 20
To get the estimated quotient think of dividing 80 by 20 or 8 by 2.
Estimated quotient = 8/2= 4
(b) 7525 is rounded to 8000
365 is rounded to 400
To get the estimated quotient think of dividing 80 by 4.
Estimated quotient = 20
Use of number in everyday life
Numbers are used immensly in our everyday life, such as measuring the length of a small object as pencil,
the distance between two given places ; the weight of an orange, the weight of a ship,
the amount of juice in a glass and the amount of water in a like.
Small lengths are measured in millimeter (mm) and centimetre (cm) while bigger
lengths are measured in metre (m) and kilometre (km).
Metre (m) is the standard unit of length and we define it as
1 m = 100 cm = 1000 mm
1 cm = 10mm
100 cm = 100 × 10 = 1000 mm
1 km = 1000 m
Also, 1 km = (1000m × 1000 mm) = 1000000 mm
Similarly, the units of weight are as under
1 gm = 1000 mg
1 kg = 1000gm
1 kg = (1000 gms × 1000) mg = 1000000 mg
For capacity or volume,
1 L = 1000 mL and 1 kL = 1000 L
1 kL = 1000 L × 1000 mL = 100000 mL
Ex. A tin of biscuits has 14 kg of biscuits. Express the weight in milligrams.
Sol. Since 1 kg = 1000 gm and 1 gm = 1000 mg
1 kg = (1000 × 1000) mg = 1000000 mg
14 kg = 14 × 1000000 mg = 14000000 mg
Ex. The population of Rajasthan is 5,64,73,122, of Goa is 13,43,998 and of Karnataka is 5,27,33,958.
What is the combined population of the three states.
Sol. Population of Rajasthan = 5,64,73,122
Population of Karnataka = 5,27,33,958. Pupulation Goa = 13,43,998
Total population of three states=
5,64,73,122 + 13,43,998 + 5,27,33,958 = 11,05,51,078 i.e. Eleven crore five lakh fifty one thousand seventy eight.
Ex. What must be added to 34,52,629 to make it equal to 6 corore.
Sol. 6 crore = 6,00,00,000
required number = 6,00,00,000 – 34,52,629 = 5,65,47,371
Ex. There are 785 students on roll in a residential public school. If the annual fee per student is Rs.62,606.
What is the total fee collected annually by the school.
Sol. Annual fee of one student = Rs.62,606
Number of student = 785
Total annual collection of fee
= Rs. 62,606 × 785
= Rs. 4,91,45,710
Ex. Find the number of pages in a book which has on an average 207 words on a page, and contains
201411 words altogether.
Sol. Number of pages = 201411÷ 207 = 973
Thus, the number of pages in the book = 973
USE OF BRACKETS
Raju bought 6 pencils from the market each of Rs. 2.
His brother Ramu also bought 8 pencils of the same type. Raju and Ramu both calculated the total cost but in their own ways.
Raju found that they both spent Rs.28 and he used the following method :
Rs. 6 × 2 + Rs. 8 × 2
= Rs. 12 + Rs. 16
= Rs. 28
But Ramu found an easier way. He did 6 + 8 = 14 and then Rs. 2 × 14 = Rs.28. The use of brackets makes this sum easy.
It can be done as follows :
Rs. 2 × (6 + 8)
= Rs. 2 × 14
= Rs. 28
Clearly we first solve the operation inside the bracket and then multiply it by the number outside.
ROMAN NUMERALS
One of the earlist systems of writing numerals is the Roman Numeral system. This system is still in use in many places.
For example, some faces of clocks show hours in Roman numerals; we use Roman numerals to write numered list; etc.
Unlike the HinduArabic numeral system, Roman numeral system uses seven basic symbols to represent different numbers. The symbols are as follows :
l = 1, V = 5, X = 10, L = 50, C = 100
D = 500, M = 1000
RULES TO FORM ROMAN NUMERALS
We can form different Roman numerals using the symbols and the following rules.
Rule1
If a symbol is repeated one after the other, its value is added as many times as it occurs. For example lll = 1 + 1 + 1 = 3
XX = 10 + 10 = 20
Rule2
The symbols l, X, C and M can be repeated up to a maximum of three times. For example l = 1,
ll = 2, lll = 3
X = 10, XX = 20, XXX = 30, C = 100
CC = 200, CCC = 300, M = 1000, MM = 2000,
MMM = 3000
Rule3
The symbols V, L and D (i.e., 5, 50 and 500 respectively) can never be repeated in a Roman numeral,
Rule4
If a symbol with a smaller value is written on the right of a symbol with a greater value, then its value is added to the value of the greater symbol. For example
Xll = 10 + 2 = 12, LX = 50 + 10 = 60
DCCCX = 500 + 300 + 10 = 810
Rule5
If a symbol with a smaller value is written on the left of a symbol with a larger value, then its value is subtracted from the value of the greater symbol.
For example,
lV = 5 – 1 = 4, lX = 10 – 1 = 9, CD = 500 – 100 = 400, Vl = 5 + 1 = 6, Xl = 10 + 1 = 11, DC = 500 + 100
= 600
Ex. Write the Roman Numerals
(a) 105 (b) 213
Sol. (a) 105 = 100 + 5 = CV
(b) 213 = 200 + 10 + 3 = CCXlll
Ex. Write in Hindu Arabic numerals :
(a) CXXXV (b) CXLl
Sol. (a) CXXXV = C + XXX + V = 100 + 30 + 5 = 135 (b) CXLl = C + XL + l = 100 + 40 + 1 = 141
OBJECTIVE TYPE
Q.1 Which of the number shown below is meaningless ?
(A) Vlll (B) XX (C) XVl (D) VXXX
Q.2 Find the difference in the place value of the digit 5 in the number 57568.
(A) 49500 (B) 45900 (C) 49000 (D) 49050
Q.3 What is the difference of 1 million and 900 tens?
(A) 100900 (B) 991000 (C) 91000 (D) 919000
Q.4 How much is 50 less than by 1 million ?
(A) 9950 (B) 99950 (C) 999950 (D) 9999950
Q.5 Which of the following numbers when rounded off to the nearest ten thousand gives 500000?
(A) 492811 (B) 495213 (C) 589200 (D) 513076
Q.6 Which one of the following is the best estimation of 5663 × 2234?
(A) 5000 × 2000 (B) 5700 × 2200 (C) 5660 × 2230 (D) 5660 × 2300
Q.7 Using the digits 1, 4, 6 and 8 each only once, how many four digit odd numbers can be formed?
(A) 4 (B) 5 (C) 3 (D) 6
Q.8 In the product of 37 and 23, what is the place value of the digit which is prime?
(A) 500 (B) 700 (C) 800 (D) 50
Q.9 How many numbers of 3digit are formed by using the digits 0, 1 and 2?
(A) 5 (B) 6 (C) 10 (D) 4
Q.10 l as a Roman numeral, am CMXClX. Break me up and then can you recognise me?
(A) 9910 (B) 999 (C) 1109 (D) 1119
Q.11 Which one of the following is the smallest numeral?
(A) 15673 (B) 15700 (C) 15198 (D) 15623
Q.12 The largest number using each of digits 5, 7, 8, 9 is :
(A) 9875 (B) 5879 (C) 8759 (D) 7589
Q.13 The successor of 49,999 is :
(A) 49,998 (B) 50,000 (C) 49,990 (D) 49,000
Q.14 The smallest 4digit number using 2, 0, 9, 5 is :
(A) 9520 (B) 0295 (C) 2059 (D) 5209
Q.15 Which digit is at thousands place in 57, 168 :
(A) 6 (B) 7 (C) 1 (D) 8
Q.16 The place value of 9 in 7690453 is :
(A) 900 (B) 9000 (C) 90000 (D) 90
Q.17 789500 comes just after :
(A) 789400 (B) 789501 (C) 789499 (D) 789498
Q.18 1 quintal = ......... kg
(A) 10 (B) 100 (C) 1000 (D) 100000
Q.19 The successor of the greatest 4digit number is
(A) 9999 (B) 9998 (C) 9909 (D) 10000
Q.20 LXV can be written in Hindu Arabic numaral as :
(A) 55 (B) 60 (C) 65 (D) 70
SUBJECTIVE TYPE
Q.1 Write the Predecessor of 7000?
Q.2 Which is greater 72389 and 72391?
Q.3 Arrange the following in ascending order:
2345, 2543, 3452, 4325, 2435
Q.4 Write the following numbers as numerals.
(a) Sixty two lakh forty five thousand six hundred thirty five
(b) Nine crore fifty eight lakh sixty one thousand eighty nine
Q.5 Write the following numbers in expanded notation.
(a) 2,84,231 (b) 52,11,568 (c) 6,04,18,517 (d) 8,91,81,213
Q.6 According to the 1991 census, the number of people who spoke the following languages were : Assamese : 13079696
Hindi : 337272114
Konkani : 1760607
(a) Write the above numbers according to the lndian and lnternational system of numeration.
(b) Write the above numbers in words according to the lndian system of numeration.
(c) Write the above numbers in words according to the lnternational system.
Q.7 Write the greatest and the smallest 5 digits numbers by using each of the digits 3, 5, 7, 0, 9 only once?
Q.8 Write the greatest and smallest numbers of 4 digits using all the digits 7, 0, 6?
Q.9 Give the approximate value by estimating.
(a) 3228 + 572 (b) 8010 – 2507 (c) 32 × 58 (d) 108 × 47
Q.10 Write the equivalent Roman numeral of each of the following HindiArabic numeral.
(a) 46 (b) 90 (c) 120 (d) 150
Q.11 Write the equilvalent Hindu Arabic numerals of the following Roman numerals.
(a) XXl (b) CCX (c) DCC (d) LXlll
Q.12 12 drums of milk have 84 litres of milk in them. Find the capacity of one drum in millilitres.
Q.13 The number of candidates appearing for class 10 board examination conducted by CBSE was 14, 58,
937 in year 2002; 16, 93, 487 in year 2003; 24, 13, 468 in year 2004 and 40, 05, 093 in year 2005.
Find the total number of candidates who appeared for the examination in these four years.
Q.14 The number of scoters produced in a year was 25, 43, 163. Out of these 16, 43, 078 were sold. How many were still left?
Q.15 A milk depot sells 657 litres of milk every day. How much milk will it sell in 1 year?
Q.16 The students of class Vl of a school collected Rs. 3, 37, 875 for Prime Minister’s Relief fund.
If each child contributed Rs.255, how many children are there in the school?
Q.17 Write all possible three digit numbers (without repeating the digits), by using the digits.
(i) 6, 7, 5 (ii) 9, 0, 2
Q.18 Use the given digits without repetition and make the smallest and the greatest four digit numbers.
(i) 2, 1, 5, 6 (ii) 7, 8, 0, 9 (iii) 4, 6, 3, 5 (iv) 8, 3, 2, 4 (v) 2, 5, 9, 0 (vi) 1, 9, 6, 3
Q.19 Make the greatest and the smallest four digit numbers by using any one digit twice :
(i)6, 3, 2 (ii) 1, 0, 6 (iii) 7, 9, 4 (iv) 2, 5, 0
Q.20 Make the greatest and the smallest 4digit numbers using any four different digits, with the condition given below:
(i) Digit 6 is always in thousands place
(ii) Digit 4 is always in hundreds place
(iii) Digit 7 is always in tens place
(iv) Digit 1 is always in ones place
(v) Digit 9 is always in thousands place
(vi) Digit 0 is always in hundreds place
(vii) Digit 5 is always in tens place
(viii) Digit 3 is always in ones place