problem solving and logic

Question

Given some numbers and you can use any of these numbers twice.

How many 2-digit numbers can be made with the numbers.

a) 3 and 4

b) 3,4 and 5

c) 3,4,5 and 6

Answer 1

In a} you can fill the 10s digit in 2 ways and then the 1s digit in 2 ways; total numbers = 4

In b) and c) the logic is the same: 9 and 16