If x, y, and k are positive numbers such that ~$10\left ( \frac{x}{x+y} \right )+20\left ( \frac{y}{x+y} \right )=k$~ and if x < y, which of the following could be the value of k?
10
12
15
18
30
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注意观察，因为x1/2. 10*1 + 20*1 > 10x/(x+y）+20y/(x+y) > 30*1/2 = 15 选D。
=10+10*[y/(x+y)],因为xy/(y+y)=1/2,10+10*[y/(x+y)]>10+10*1/2=15; 因为x,y都是整数，所以y+x一定大于y，y/(x+y)
解答：https://gmatclub.com/forum/if-x-y-and-k-are-positive-numbers-such-that-x-x-y-10-y-x-y-128231.html
why?
x < y,
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注意观察，因为x1/2.
10*1 + 20*1 > 10x/(x+y）+20y/(x+y) > 30*1/2 = 15
选D。
=10+10*[y/(x+y)],因为xy/(y+y)=1/2,10+10*[y/(x+y)]>10+10*1/2=15; 因为x,y都是整数，所以y+x一定大于y，y/(x+y)
解答：https://gmatclub.com/forum/if-x-y-and-k-are-positive-numbers-such-that-x-x-y-10-y-x-y-128231.html
why?
x < y,