How many 5-digit numbers are there, where no two digits are the same?

9*9*8*7*6

If we include leading zero the answer is obviously 10*9*8*7*6. 1/10 of these numbers has a leading zero, the other 9/10 do not have leading zero and thus they are five-digit numbers. So, the result is

10*9*8*7*6*(9/10) = 9*9*8*7*6