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x, y, and k are positive numbers

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by ska7945 » Tue Aug 12, 2008 5:15 pm
If x, y, and k are positive numbers such that [x/(x+y)]10+[y/(x+y)]20=k and if x < y, which of the following could be the value of k?

A. 10
B. 12
C. 15
D. 18
E. 30

answer D
i solved by substituting. Is there any other way?
thx.
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by Ian Stewart » Tue Aug 12, 2008 8:13 pm
Yes, the question can be solved much more quickly- the expression just represents a weighted average. I went into more detail here:

www.beatthegmat.com/tough-algebra-equation-t13568.html
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
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