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
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In Total, 10*9*8*7*6 combinations.
