How many four-digit personal identification numbers (PIN) are possible if the PIN cannot begin with a 0 and the PIN must be an odd number?

The first digit has 9 choices (1 through 9), since the PIN cannot begin with 0. The last digit has 5 choices (1, 3, 5, 7, or 9) so the PIN is odd, and each of the two middle digits has 10 choices. Thus, 9 × 10 × 10 × 5 = 4,500, which is choice B, “4,500.”

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