There's a word problem I don't know how to solve. The answer is 70 soybeans, 30 corn, but I don't know how to get to that answer:

"Mrs. Griffin wants to plant soybeans and corn on 100 acres of land. Soybeans require 6 hours of labor per acre, and corn requires 8 hours of labor per acre. If Mrs. Griffin has 660 hours available, how many acres of each crop should she plant?"