Hi Rich,

You can choose the first digit in two ways, it can be 2 or 5. Whatever value you choose for the first digit you have two choices for the second digit. Hence there are 2 ×2 = 2² = 4 choices for two digits. they are

22
25
52
55

From each of these two digit numbers you can construct a three digit number in two ways. You can put a two in the leftmost position

222
225
252
255

or a five in the leftmost position

522
525
552
555

This give a total of 2 × 2 = 2³ = 8 three digit combinations.

Using this same procedure you can discover the number of six digit combinations and generate a list of them,

Penny