k=[(x+y)/(x+y)]*10+10*y/(x+y)=10+10y/(x+y)
x、y、k都是正数,且xy/(x+y)>y/(y+y)=1/2
20>k>15
所以选D
GMAT数学题If x,y,and k are positive numbers such that {X/(X+Y)}
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