2007 AMC 12A Problem 16

Attempt Problem 16 of the 2007 AMC 12A below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2007 AMC 12A solutions, or check the answer key.

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16.

How many three-digit numbers are composed of three distinct digits such that one digit is the average of the other two?

9696

104104

112112

120120

256256

Answer: C
Concepts:arithmetic sequencedigitscasework
Difficulty rating: 1630
Solution:

The three distinct digits form an increasing arithmetic progression. Counting by common difference: 88 with difference 1,1, 66 with difference 2,2, 44 with difference 3,3, and 22 with difference 4,4, for 2020 sets.

Of these, 44 sets contain 00 (namely {0,1,2},\{0,1,2\}, {0,2,4},\{0,2,4\}, {0,3,6},\{0,3,6\}, {0,4,8}\{0,4,8\}); each yields 22!=42\cdot 2!=4 valid numbers since 00 cannot lead.

The other 1616 sets each yield 3!=63!=6 numbers. The total is 44+166=112.4\cdot 4+16\cdot 6=112.

Thus, the correct answer is C.

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