Price Quantity TR TC Profit MR MC
6 0 0 3 -3 - -
6 1 6 5 1 6 2
6 2 12 8 4 6 3
6 3 18 12 6 6 4
6 4 24 17 7 6 5
6 5 30 23 7 6 6
Consider table above. If my understanding is correct, optimal quantity equals 5, because at this point MR = MC. But after producing 5th widget, profit does not increase. Then why is producing 5 widgets considered to be profit maximizing quantity, but not 4?