Afour-digit serial number is to be created from the digits 0 through 9. how many of these serial numbers can be created if 0 can not be the first digit, no digit may be repeated, and the last digit must be 5?

For the answer to the question above,
the 1st digit has 9 possible digits 2nd digit: 9 possible digits
3rd digit: 8 possible digits
4th digit: 1 possible digits (number 5 only)
Number of serial numbers = 9 * 9 * 8 * 1
= 648
I hope this helps. Have a nice day ahead!

Logical equivalent of the give question is its contrapositive