We will learn how to solve different types of word problems on digits and numbers.
1. The number of digits used in numbering each page of a book of 150 pages is
(a) 342
(b) 348
(c) 328
(d) 322
Solution:
Required number of digits = (9 × 1) + (90 × 2) + (150  99) × 3
= 9 + 180 + 153
= 342
Answer: (a)
Note: Number of 1 digit numbers = 9
Number of 2 digit numbers = 90
2. If the digits of a twodigit number are interchanged, the number so formed is 18 more than the original number. The sum of digits is 8. What is the thrice value of original number?
(a) 159
(b) 78
(c) 105
(d) None
Solution:
Let, the unit’s and ten’s digit be ‘a’ and ‘b’ respectively.
Therefore, a + b = 8 and a  b = 18 ÷ 9 = 2
Since, a = (8 + 2) ÷ 2 = 5 and b = 5  2= 3
Therefore, the required value is 3 × 35 = 105
Answer: (c)
3. A twodigit number is eight times the sum of its digits. If 45 is subtracted from the numbers, the position of digits get interchanged. The number is:
(a) 56
(b) 83
(c) 72
(d) None
Solution:
Two digits differ by = 45 ÷ 9 = 5 and the number is divisible by 8.
Now according the options,
The required number is 72
Answer: (c)
Note: The difference of a two digit number and the number obtained by writing it in reverse order is equal to nine times of difference of digits.
4. Find the total numbers of threedigit numbers, which are exactly divisible by six?
(a) 340
(b) 126
(c) 190
(d) 150
Solution:
Total number of 3digit numbers from 100 to 999 is 900.
Therefore, the least 3digit number divisible by 6 is 6 × 17 = 102 and greatest number is 6 × 166 = 996
Therefore, required number of numbers = 166  17 + 1 = 150
Answer: (d)
5. A mathematics book containing 20 pages. One sheet is missing among them. Sum of the page numbers of the remaining pages of the mathematics book is 195. The numbers written on both the sides of the missing sheet of the mathematics book must be
(a) 9, 10
(b) 11, 12
(c) 5, 6
(d) 7, 8
Solution:
Sum of the page numbers of the book = 1 + 2 + 3 + ................ + 19 + 20 = (20 × 21)/2 = 210
The sum of page numbers of missing sheet = 210  195 = 15 = (7 + 8)
Therefore, required page number of missing sheet = 7 and 8.
Answer: (d)
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