dancinggeometry wrote:

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

Ans - D

This question is very well explained on

MGMAT forum. Here is the explaination:

1. Solving the equation a little will yield following that represent a weighter Avg. equation

[10X + 20Y]/(X + Y)

2. Now, 10 < K < 20. This will eleminate option A & E

3. Since, Y > X and 20 > 10 therefore K should be quite close to 20. This eleminates B & C and D is the best guess

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