(10x + 20 y)/(x+y) = k


10x + 20y = kx + ky

10x - kx = ky - 20y

x(10-k) = y (k-20)

x/y = (k-20)/(10-k)


Insert the values for k

A) 10----k cannot be 10 since denominator will be 0. Eliminate.
B) 12----
x/y = 4/1
4y=x, this cannot be possible since y>x and both the numbers are positive. Eliminate

C) 15----
x/y=1/1, x=y, same reason as above. Eliminate.

D) 18----
x/y=1/4, 4x=y, this is the only possible answer which satisfies all the above conditions.

E) 30----

x/y = -(10/20). x, y and k are all positive numbers, Eliminate.

Hence Answer is D.

Whats the OA?

That's clear there is an analogy with weighted average, but given I did not manage to use it to solve the question, I did that (parallel to Chase):

10x + 20y = (x+y)*k

A) 10
10y=0
That's wrong
B) 12
8y=2x
No because y>x
C) 15
5y=5x
No because y>x
D) 18
2y=8x
It seems nice, let's check if E is wrong
E) 30
-10y=20x
Impossible x>0 and y>0

That's D

Thanks for input. OA is D.