Answer: The firms profit would have been higher by $800 if it had not produced the last 400 units.
Step-by-step explanation: The optimal quantity of a profit maximising producer is given by the condition,
MR=MC
It is only when the two are equal that the profit of the firm is maximised. At any quantity lower than the optimal quantity, MR>MC, so it is profitable for the producer to produce more units.
At any quantity above the optimal quantity, MR<MC, so it will be profitable to the producer to sell less units.
Here, as we can observe that the last 400 units increase revenue by only $4000 but cost by $4800. This means that MR of $4000 is less than MC of $4800. So it will be profitable for the producer to sell less units to be at the optimal level.