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主题：[The Official SAT Question of the Day™] November 11th Tuesday

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Add 8x to 2x and then subtract 5 from the sum. If x is a positive integer, the result must be an integer multiple of
A. 2
B. 5
C. 8
D. 10
E. 15
最后由 jasontomonaga 于 2008-11-11 17:56:47编辑

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SAT题目：http://apps.collegeboard.com/qotd/question.do

楼主 Date: 2008-11-11 17:55:34

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Hint : It may be helpful to write out mathematically the expression that is given in words in the question. Then see which of the answers must be a factor of the number represented by the expression in x.

沙发 Date: 2008-11-11 18:37:58

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Correct Answer: B

Here's Why: The mathematical expression given in the question is 8x + 2x – 5, which is equivalent to 10x – 5. The expression 10x – 5 can be factored as 5(2x – 1). For every positive integer x, 5(2x – 1) must be a multiple of 5. If x = 1, then 5(2x – 1) = 5, which is not an integer multiple of 2, 8, 10, or 15. Therefore, the correct answer is 5.
Difficulty: Medium
Question Type: Standard Multiple Choice
(Mathematics)

板凳 Date: 2008-11-11 18:38:38

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