一道题目始终没想通，gwd

The R studendts in a class agree to contribute equally to buy their teacher a birthday present that costs y dollars. If x of the students later fail to contribute their share, which of the following reprsents the additional number of dollars that each of the remaining students must contribute  in order to pay for the present?

a) y/r

b) y/ r-x

c) xy/r-x

d) xy/r(r-x)

e) y/r(r-x)

我选的是C，为什么GWD上正确答案是D？ 没想通

((y/r)*x)/(r-x)=xy/(r*(r-x))

so choose D

哦，对的，我把他算成了每人交Y元

谢谢