$7 $25 Cost / price (dollars) 57 Daily rate of production Marginal cost Average variable cost Total variable cost = area under MC $25 7 At $25, marginal cost meets price at 7 units; variable cost = area under MC = $70. The seventh unit's $22 marginal cost still clears the $25 price;the eighth's $30 would not.

Drag the output handle along the axis, or the price line up and down (or focus either and use the arrow keys). The output snaps at the profit-max unit; the price snaps at $25 and the $7 shutdown floor.