Final answer:
The new market price after a new firm enters a Cournot duopoly charging $20 will likely be below $20, due to increased supply and competitive pricing driving the market towards a new equilibrium with zero economic profits.
Step-by-step explanation:
When a new firm enters a market that is initially served by a Cournot duopoly charging a price of $20, the introduction of the new firm will increase the total quantity supplied in the market, assuming all firms can coexist and the market doesn't have high barriers to entry like in a monopoly. Due to the increase in supply, the market price is driven down, as per basic principles of supply and demand. Therefore, the new market price will likely be below $20, as the existing firms will likely lower their prices in response to the added competition, until all firms in the market reach a new equilibrium where no new firms enter and none leave, potentially earning them zero economic profits in the long run.