2005 AMC 12A Problem 10

Attempt Problem 10 of the 2005 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 2005 AMC 12A solutions, or check the answer key.

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10.

A wooden cube $n$ units on a side is painted red on all six faces and then cut into $n^3$ unit cubes. Exactly one-fourth of the total number of faces of the unit cubes are red. What is $n?$

$3$

$4$

$5$

$6$

$7$

Answer: B

Concepts:cube geometry surface area ratio and proportion

Difficulty rating: 1430

Solution:

The $n^3$ unit cubes have $6n^3$ faces total. The red faces are exactly the surface of the original cube, $6n^2$ of them.

Setting the red fraction to one-fourth, $\dfrac{6n^2}{6n^3} = \dfrac{1}{n} = \dfrac{1}{4},$ so $n = 4.$

Thus, the correct answer is B.

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