Kenneth S. answered • 05/04/18

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First case: all 3 symbols are letters: 26^3 = 17,576 possible

Second case: 2 letters & one digit: 10(26^2) = 6,760 by considering the one digit to be qt extreme right.

But there are two other possibilities--one digit in middle & one digit at extreme left. There this accounts for 3(6760) = 20,280 possible "second case" arrangements.

final answer: 17,576 + 20,280 which agrees with your quoted answer.