If x, y, and k are positive numbers such that ( (x/(x+y) (10) + (x/(x+y) (20) ) = k and if x < y, which of the following could be the value of k?
- 1
10
12
15
18
30
Source: OG 12th E; PS #148
palvarez wrote:(10x+20y)/(x+y) = k, which is an integer
10 + (10y/(x+y)) = k
k > 10
10y/(x+y) = 10/(1+(y/x))
y/x's max value 1.
10/(1+(y/x))'s min value = 5
therefore, k > 15
20 - (10x/(x+y)) = k
here, k < 20
We got one value in that range. 18
10y = 8x+8y
y = 4x, which is consistent with x < y.