Question

how many two digit numbers contain the number 7​

Answer 1

Answer:

There are 18 two digit numbers containing 7.

Solution:

7 is a unit digit and if we add 10 with it, it results a two digit number 17, just by changing the digit in the tenth place, contains 7.

In this process, we find numbers 17, 27, 37, 47, 57, 67, 77, 87 and 97. It counts to 9.

We must not forget about the numbers lying between 70 and 79, all of them containing 7 and they are 70, 71, 72, 73, 74, 75, 76, 77, 78 and 79. It counts to 10 but since we have already counted 77 in this previous paragraph, the count is (10 - 1) = 9 now.

Hence, the count of two digit numbers containing 7 is

= 9 + 9 = 18