Answer:
Step-by-step explanation:
Let x represent the number of tons per acre harvested before new technology. The problem statement gives us the relation ...
320(x +4) = 400x +640
320x + 1280 = 400x + 640 . . . . eliminate parentheses
640 = 80x . . . . . . . . . . . . . . . . . . add -320x-640
8 = x . . . . . . . . . . . . . . . . . . . . . . . divide by the coefficient of x
Before new technology, 8 tons per acre were collected; afterward, 12 tons per acre were collected.
_____
The field yield was 8 tons per acre before new technology:
320 acres × 8 tons/acre = 2560 tons
400 acres × 8 tons/acre = 3200 tons
After new technology, the harvest was ...
320 acres × 12 tons/acre = 3840 tons
400 acres × 12 tons/acre = 4800 tons
The highlighted numbers are the ones being compared, so must differ by 640.