Question

How many 5-digit numbers can be formed from the integers 1, 2, ... , 9 if no digit can appear more than twice?

Answer 1

5 digit number with no repeats
1st digit has 9 possibilities
2nd digit has 8 possibilities
3rd digit has 7 possibilities
4th digit has 6 possibilities
5th fight has 5 possibilities
Therefore the answer is:
9 x 8 x 7 x 6 x 5 = 72 x 210 = 15,120