2008 AMC 12A Problem 5

Below is the professionally curated solution for Problem 5 of the 2008 AMC 12A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2008 AMC 12A solutions, or check the answer key.

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Concepts:algebraic manipulationparity

Difficulty rating: 1100

5.

Suppose that 2x3x6\dfrac{2x}{3} - \dfrac{x}{6} is an integer. Which of the following statements must be true about x?x?

It is negative.

It is even, but not necessarily a multiple of 3.3.

It is a multiple of 3,3, but not necessarily even.

It is a multiple of 6,6, but not necessarily a multiple of 12.12.

It is a multiple of 12.12.

Solution:

Combining the fractions, 2x3x6=4xx6=x2. \dfrac{2x}{3} - \dfrac{x}{6} = \dfrac{4x - x}{6} = \dfrac{x}{2}.

This is an integer exactly when xx is even. The example x=4x = 4 is even but not a multiple of 3,3, which rules out every other statement.

Thus, B is the correct answer.

Problem 5 in Other Years