Question
A teacher asks each student to pick a four or five digit password using the numbers 0 to 9. If repetition is not allowed, how many different passwords are possible?
Answers
oobleck
1st digit: 10 choices
2nd digit: only 9 choices left.
and so on. So the total choices are
10*9*8*7*6 + 10*9*8*7
2nd digit: only 9 choices left.
and so on. So the total choices are
10*9*8*7*6 + 10*9*8*7