The first thing we are going to do for this case is define variables.
We have then:
x: Amount of soybean meal
320 - x: quantity of cornmeal
We now write the equation that models the problem:

Rewriting we have:

From here, we clear x:

Then, the amount of Soybean meal is:

The amount of cornmeal is:
Answer:
cornmeal: 280 lb
Soybean: 40 lb.