13. Refer to the following table. Output Total variable oost($) ' Total cost($) 0 0 " 100 1 100 200 2 ' 140 ' 240 3 190 290 it ' 556 '1 336 5 ' 320 " 420 6 400 500 7 490 ' 590 8 590 __ 690 Suppose the rm has maximized profit at the output of 8 units, what is the marginal revenue earned from the eighth unit