Answer: 20 units.
Step-by-step explanation:
Given that,
Inverse demand curve: P = 420 - 2Q
There are five firms and each of the firm has a constant marginal cost.
Marginal cost (MC) = 20
Profit maximizing output is produced by the firms is at a point where the marginal cost is equal to marginal revenue.
P = 420 - 2Q
Total revenue(TR) = PQ
= 420Q - 2

Differentiating TR with respect to 'Q'
Marginal revenue(MR) = 420 - 4Q
MR = MC
420 - 4Q = 20
Q =

Q = 100
Therefore, output produced by the industry is 100 units.
Per-firm production =

=

= 20 units
Hence, each firm produces 20 units.