Let the diet contain x and y packets of foods P and Q respectively. Therefore, x ≥ 0 and y ≥ 0
The mathematical formulation of the given problem is as follows. Maximize z = 6x + 3y … (1) subject to the constraints,

The feasible region determined by the system of constraints is as follows.

The corner points of the feasible region are A (15, 20), B (40, 15), and C (2, 72).
The values of z at these corner points are as follows.

Thus, the maximum value of z is 285 at (40, 15).
Therefore, to maximize the amount of vitamin A in the diet, 40 packets of food P and 15 packets of food Q should be used. The maximum amount of vitamin A in the diet is 285 units.