Detail

As we learned before, unlike other costs, fixed costs will be the same at any level of output. Therefore, in general, it doesn't matter at what Q we compute them. However, in this graph the AFC curve isn't shown, so we have to figure out AFC as the difference between ATC and AVC. At an output level of 10 we happen to have both numbers, so this is a reasonable way to compute fixed costs. In particular, we can compute it as 10 x (20 - 15) = 50. This is represented by the shaded area.

Look at the graph at an output level of 7. When output is 7, ATC is 14. Even though it isn't marked we can conclude that AVC at 7 must be just slightly under seven, because fixed costs must be the same everywhere, and 7 x (14 - 7) = 49, just slightly less than 50.

12. Consider the profit maximizing perfectly competitive firm shown in Figure 3. This firm's total fixed costs will be:

$150. At an output of 10, this is total variable costs.
$20. At an output of 10, this is the average costs per unit.
$15. At an output of 10, this is the variable costs per unit.
$50.
$200. At an output of 10, this is the total cost.