Can I have 3 unknowns in this system of equations? Or just two? Any help is much appreciated.

A mix which contains 40% barley and 60% oats is added to a mix that contains 20% barley, 50% oats and 30% corn to get 5 tons of feed which is 35% barley. How much of each mixture do you need and what percent corn is the final result.

1. Define unknowns and set this up as a system of three equations and three unknowns.
2. Solve the system using row operations.

i think, 2 variable is good enough.

let x be the first mix, and y be the 2nd. mix
x + y = 5
0.4x + 0.2y = 0.35*5 = 1.75
solving
x = 3.75
y = 1.25
30% corn = 0.3*1.25 = 0.375
0.375 / 5 = 7.5% of corn in the final result