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Problem 20 - Entrance Test

How many 3-digit numbers have exactly two digits equal to 7?

Correct: E

Cases: 77x, 7x7, x77 with x≠7. Each has 9 choices for x (0-9 except 7), but x≠0 in x77. So 77x:9, 7x7:9, x77:8 (x=1-9,7). Total 9+9+8=26.