5. You are the manager of a monopoly. A typical consumer's inverse demand function for your firm's product is P= 250 - 40Q, and your cost function is C(Q) = 10Q. a. Determine the optimal two-part pricing strategy.

Given answer by my professor:

The efficient consumption is the level when demand=marginal cost. The marginal cost is $10 per unit. Let 250-40Q=10, we have Q=6. In order to induce consumers to buy 6 units, the unit price must be p=250-40*6=$10. The fixed fee then should be (250 - 10)(6)(0.5) = $720.

Why is this formula for fixed fee? I don't understand how I can make this calculation for fixed fee.