x + y = 180
Y = wheat, x = oats
Y = x + 20. In the 1st equation, get y by itself getting y = 180 - x.
So 180 - x = x + 20. Subtract x on both sides to get 180 - 2x is 20. Subtract 180 to both sides is -2x = -160. Divide -2 to both sides is x = 80. So 80 + 20 is 100 wheat, and 80 oats of acres.