Question

A lock on a fence door has a 4 digit combination. Each digit can be any number

between 0 – 9. The only restriction is that all 4 characters cannot be the same
(e.g. 1111, 2222, 3333, etc.). How many combinations are possible?




504

512

10000

9990

Answer 1

9990

Explanation:

Each number combination is 0 to 9. For each combination, it is 10^X. In your problem there are 4 digits. That means it is it is 10^4 = 10000, but because there is a stipulation that the numbers cannot be identical then we have to remove it from the possible combinations. So there are 10 possible identical number combinations. 10000 - 10 = 9990