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GMAT数学题If x,y,and k are positive numbers such that {X/(X+Y)}

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GMAT数学题
If x,y,and k are positive numbers such that {X/(X+Y)}*(10) + { Y/(Y+X)}*(20) = k and if x < y,which of the following could be the value of
A.10
B.12
C.15
D.18
E.30
但是我算得出K
GMAT数学题If x,y,and k are positive numbers such that {X/(X+Y)}
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
再问: 你的做法我明白了,但是我这样做为什么不对呢?

原式=(10X+20Y)/(X+Y)=K ;
10X+20Y=KX+KY ;
X(10-K)=Y(K-20) ;
因为XK-20 ;
得出K