An aircraft with 100 seats serves passengers through two types of fares: full ($550) and discount ($250).

Question:

An aircraft with 100 seats serves passengers through two types of fares: full ($550) and discount ($250). Extra passengers have $50 marginal cost. Demand for discount tickets is unlimited, while demand for full-fare tickets is evenly distributed between 11 and 30 seats. How many seats should be protected for full-fare passengers and not authorized for release to the discounted $250 segment?