Final answer:
The expected cost of buying the second-hand computer from the charity is $500.
Step-by-step explanation:
To determine the expected cost of buying the second-hand computer from the charity, we need to consider the fixed costs and marginal costs provided in the question. The fixed cost of the computer company is $250. The marginal cost for the second computer is $250. Therefore, the expected cost of buying the second-hand computer from the charity would be the fixed cost plus the marginal cost, which is $250 + $250 = $500.