Question

how many 9's between 1 and 100

Answer 1

First number = 9

Last number = 99

So,

Difference between them is 9

Now,

☛ a + (n-1)d = 99

☛ 9 + (n-1)9 = 99

☛ 9 + 9n - 9 = 99

☛ 9n = 99

☛ n = 11

Hence, There are 11 9's number between 1 and 100