The number of 3-digit telephone area codes that can be made if repetitions are not allowed is?

a. 100

b. 720

c. 1,000

d. 504

Each of the 3 digits can be from 0 to 9, so each of the digits can have 10 values.

10 * 10 * 10 = 1,000

However, if repetition is NOT allowed, then the first digit can be ANY digit (10 possibilities). The second digit can be ANY digit except what the first digit is (9 possibilities). The third digit can be ANY digit except what the first digit is or what the second digit is (8 possibilities).

Total area codes = 10 * 9 * 8 = 720

Answer is b