Question

Area codes for phone numbers are 3 digits long. How many different area codes can be made from the digits 0 through 9 if no digit is used more than once in any single area code? 27 30 720 1,000

Answer 1

For the first digit there are 10 options. For the second digit, the number used in the first digit can not be reused leaving 9 options. The third digit had 8 options.

10*9*8=720