SOLUTION: how many three digit numbers can be formed from 1,2,3,4,5,6,7,8,9 if repetition of digits are not allowed? a) 60 b) 343 c) 504 d) 729 e) none of the choices.

There are 9 ways to pick the 1st digit

For each of those 9 ways there are 8 ways to choose the second digit.
That's 9×8 or 72 ways to pick the first two digits.

For each of those 72 ways there are 7 ways to choose the third digit.
That's 72×7 ways or 504 ways to pick all three digits.

Edwin