2015 AMC 10B Problem 25

Attempt Problem 25 of the 2015 AMC 10B 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 2015 AMC 10B solutions, or check the answer key.

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25.

A rectangular box measures a×b×c,a \times b \times c, where a,a, b,b, and cc are integers and 1abc.1\leq a \leq b \leq c. The volume and the surface area of the box are numerically equal. How many ordered triples (a,b,c)(a,b,c) are possible?

4 4

10 10

12 12

21 21

26 26

Answer: B
Concepts:Diophantine EquationSimon’s Favorite Factoring Trickcasework
Difficulty rating: 2010
Solution:

The condition is abc=2(ab+ac+bc).abc=2(ab+ac+bc). Since abc6bcabc\le6bc, we have a6a\le6. Also a=1a=1 and a=2a=2 give no positive solutions, so test a=3,4,5,6a=3,4,5,6.

For a=3a=3, (b6)(c6)=36(b-6)(c-6)=36, giving (b,c)=(7,42),(b,c)=(7,42), (8,24),(8,24), (9,18),(9,18), (10,15),(10,15), (12,12)(12,12). For a=4a=4, (b4)(c4)=16(b-4)(c-4)=16, giving (5,20),(6,12),(8,8)(5,20),(6,12),(8,8).

For a=5a=5, (3b10)(3c10)=100(3b-10)(3c-10)=100, and the only solution with abca\le b\le c is (b,c)=(5,10)(b,c)=(5,10). For a=6a=6, (b3)(c3)=9(b-3)(c-3)=9, and the only solution with abca\le b\le c is (6,6)(6,6).

The total number of triples is 5+3+1+1=105+3+1+1=10.

Thus, the correct answer is B.

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