Matchpoint company produces 3 types of tennis balls: heavy duty, regular, and extra duty, with a profit contribution of $24, $12, and $36 per gross (12 dozen), respectively. The linear programming formulation is: max. 24x1 + 12x2 + 36x3 subject to: .75x1 + .75x2 + 1.5x3 _ 300 (manufacturing) .8x1 + .4x2 + .4x3 _ 200 (testing) x1 + x2 + x3 _ 500 (canning) x1, x2, x3_ 0 where x1, x2, x3 refer to heavy duty, regular, and extra duty balls (in gross).

By how much would the profit contribution of regular balls have to increase to make it profitable for matchpoint to start producing regular balls?