Lei X. answered • 11/20/19

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Teaches Math and Mandarin for age 14 and above

You have to count the number of integers in two situations:

- all three digits are different. In this case you have a permutation of 3 out of 9 which is 9*8*7=504 integers
- one digit appears twice. In this case you first choose 2 digits from 9 which is 9*8/2=36 ways. and each time you have 3 ways to permute. For example if you choose 1 and 2 you can get 112,121,or 211 three integers. So in total you have 3*36=108 integers.

add 1 and 2, 504+108=612.