



 
Hi Camille, Suppose you wanted to list all such numbers. Starting from left to right the first digit is a 3. For the next digit you have 9 choices, 0, 1, 2, 4, 5, 6, 7 ,8 or 9. Hence, so far, your list is
Regardless of which 2 digit number you have you can extend it to a 3 digit number in 8 ways. Thus, for example, if you have the 2 digit number 37 you can extend it to
Since each of the nine, 2 digit numbers can be extended to eight 3 digit numbers your list now has $9 \times 8 = 72,$ 3 digit numbers. What happens when you add a fourth digit? Penny  


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