Question

How many two digits numbers are divisible by 3?

Answer 1

Two digits numbers divisible by 3 are 

12,15,18, .......99.

a = 12, d= 15 -12=3

implies that

T_n =99 

implies that

a + (n -1) d = 99

implies that

12 +(n -1)3 =99

implies that 

n = 30

numbers of two digit numbers divisible by 3 are 30.