Create a 4-digit number from 0 to 9 digit, each parent will need to have duplicate numbers (that is, repeated 2 or more) are all a matter of how much.Why all the way from the "a" minus "for which there is no duplicate numbers."-All the way (thousands cannot be 0).Is equal to?There is no way to duplicate numbers-at all times with.Therefore, The number of the desired method (duplicate numbers are used).
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