Soybean meal is 14% protein; cornmeal is 7% protein. How many pounds of each should be mixed together i order to get 280-lb mixture that is 8% potein?

solve for x ...

.14x + .07(280-x) = .08(280)

I suggest multiplying each term by 100 to get
14x + 7(280-x) = 8(280)