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 heavy duty balls have to decrease before matchpoint would find it profitable to change its production plan?