If x, y, and f are positive numbers such that 10*x/(x+y)+ 20*y/(x+y)=f and if x is less than y, which of the following could be the value of f?
Choose the correct choice from the following answers
A) 10	B) 12	C) 15	D) 18	E) 30
Answer choice C: f= 15   (10x + 20y)/(x+y) = 15   10x + 20y = 15x + 15y   5y = 5x   y = x   Doesn't work because the problem states that x&lt;y.   We need y to be larger, so let's try answer choice D: f=18   10x + 20y = 18x + 18y   2y = 8x   y/x = 8/2   Success! x&lt;y.   The correct answer is 18.
<result>D</result>
If x, y, and f are positive numbers such that 10*x/(x+y)+ 20*y/(x+y)=f and if x is less than y, which of the following could be the value of f?