gwd-11-36

uspresident

Q36: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 x=2,y=8

E. 30

这个题我是凑数字的，正常应该怎么算？

daoying

10x/(x+y) +20y/(x+y)] ＝(10x+10y)/(x+y)+10y/(x+y)=10+10/(x/y+1)

因为0<x<y => 0<x/y<1

=> 1<x/y+1<2

=> 5<10/(x/y+1)<10

=> 15<10+10/(x/y+1)<20

只有D符合条件