Methods of solving LPP: a graphical method

You have the following linear programming problem:

$$Maximize \ z = 4x_1 + 3x_2$$

Storage capacity constraint:

$$x_1 + x_2 \le 60$$Labor constraint:

$$2x_1 + x_2 \le 80$$Raw material constraint:

$$3x_1 + x_2 \le 120$$What is the marginal gain if we invested an additional 20 units each in capacity and labor?

Output format:

100