solution:

The small pizza has a diameter of 10" and therefore an area proportional to 102.

The large pizza has a diameter of 12" and therefore an area proportional to 122.

If the pizzas are priced in accordance with their areas, then

Price of large pizza / Price of small pizza = (12 / 10)2 = 1.44 or

Price of large pizza = 1.44 × 3.99 = 5.75.

Thus, Discount on large pizza = [1 − (5.17 / 5.75)] × 100 = 10 %.