Question

A combination lock has 5 different numbers. If each number can only be used ONCE, how many different combinations are possible?

Answer 1

Hello,

The first digit may be 0,1,2..,9 thus 10 possibilities
The second digit may be 0,1,2,3...,9 but not equal to the first thus 9 possibilities
The 3th digit may be 0,1,2..,9 but not the first and not the 2th =>8 possibiliteis
...
In Total, 10*9*8*7*6 combinations.