Answers

2016-07-13T21:02:51+08:00
We have a possibility of 9 * 9 * 8 * 7 = 4536

Thousands: We cannot have 0 so we only have one choice
Hundreds: We have 10 - 1 or 9 choices since we already used one for ones
Tens: We have 8 choices left
Ones: We have 7 choices left

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