Answer:
Fertiliser should cost less than $20 per pound.
Explanation:
Marginal analysis states that the marginal benefit of taking an action should put weight the marginal cost incurred for taking the action.
The additional pound of fertiliser resulted in a crop gain of $100-$80= $20
So the one pound of fertiliser added should be less than $20 for the additional of fertiliser to be economically profitable.
If for example the amount of one pound of fertiliser is more than $20 the business will start to make a loss.