Answer:
$100
Explanation:
C(x) = F + 20x,
where,
C(x) is total cost,
F is a fixed cost,
and x is the level of output
At p = 30,
x = 60 - 30
 = 30
Total revenue = px
            = 30 × 30
            = 900
Total cost = F + 20x
        = F + 20 × 30
        = F + 600
At zero economic profit,
TR = TC
F + 600 = 900
F = 300
Total cost = 300 + 20x
Marginal cost = 20
Demand, p = 60 - x
Total revenue = px
            = (60 - x) x
            = 60x - x^2
Marginal revenue = 60 - 2x
Equilibrium: MR = MC
60 - 2x = 20
x = 20
p = 40
Profit = TR - TC
     = 40 × 20 - 300 - 20(20)
     = 800 - 300 - 400
     = $100