Question:
How many 2 - digit numbers are divisible by $7 ?$
Solution:
The first 2 digit number divisible by 7 is 14, and the last 2 digit number divisible by 7 is 98, so it forms AP with common difference 7
$14, \ldots, 98$
$98=14+(n-1) \times 7$
$n=22$