Question

How many 4 digit numbers are there that do not contain 3 or 6?

Answer 1

Let the 4-digit number be abcd

a can't be 0,3 or 6

b,c & d can't be 3 or 6

So ways for filling:

a = 7

b = 8

c = 8

d = 8

Total numbers = 7*8*8*8 = 3584

Answer 2

Answer:

For the 3-digit numbers with no threes: Their first (hundreds) digit can be chosen in just 8 ways (no 0 and no 3), their second digit in just 9 (no 3 remember), and their third digit in 9 ways. So there are 8 x 9 x 9 of these. That’s 648 altogether.