Hi Steve,

Think about you would construct such a string of "digits". To choose the first digit you have 16 choices, any of the numbers from 1 to 16. Once you have the first digit chosen you have 15 choices for the next digit. Thus you have 16 × 15 ways to choose the first two digits. Now, regardless of your choices so far you have 14 ways to choose the third digit so each of the two digit numbers you have can be turned into a 3 digit number in 14 ways. Thus you have 16 × 15 × 14 ways to choose the first two digits.

Continue.

Penny