66
There are 210 4 digit combinations and 5040 different 4 digit codes.
In other words, how many 4 digit combination locks are there using the digits 0-9 on each wheel. There are 10×10×10×10 = 10⁴ = 10,000 such combinations.
5,040
There are 9C3 = 10*9*8/(3*2*1) = 120 of them.
There are 1,120,529,256 combinations.
9
66
45 In combinations, the order of the digits does not matter so that 12 and 21 are considered the same.
There are 5,040 combinations.
10C6 = 10*9*8*7/(4*3*2*1) = 210 combinations.
There are 210 4 digit combinations and 5040 different 4 digit codes.
In other words, how many 4 digit combination locks are there using the digits 0-9 on each wheel. There are 10×10×10×10 = 10⁴ = 10,000 such combinations.
9
I believe the answer would be 10^9 or 1,000,000,000
If every number can be used as many times as you like, there are 104 = 10000 different combinations. If each number can only be used once, there are 9!/(9 - 4)! = 5040 combinations.
There are ten combinations: one each where one of the ten digits, 0-9, is excluded.