How many cups of grape juice containing 10% fruit juice and berry punch containing 20% fruit juice must be added together to create 12 cups of punch with 18% fruit juice?

It is 2.4 cups of 10% fruit juice and berry punch and 9.6 cups of 20%fruit juice:
let x to be 10% fruit juice and berry punch and y= 20% fruit juice
.x+.2y=2.16
x+y=2.4
use substitution property or elimination property to solve the problem:
x=2.4 and y=9.6

The answer is why would a teacher ask that

Ibelieve the answer is b. y=0 would make the line horizontal across the x.