[問題]天山03-36

[applepeko]

If x,y and k are positive number 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

我算不出來這題,有人可以替我解答嗎??謝謝呢