请教大牛对此题的解答

faytastic

If x, y, and k are positive numbers such that ( )(10) + ( )(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

kazemi

没看懂题目，那两个空括号代表什么呀