Problem: The producer of a cosmetic product needs to decide the optimal price to charge and the optimal quantity to supply in the market. The demand function of the cosmetic product is given as QD = 40 - 2P, while the supply function of the cosmetic product is given as QS = 2P where P is price, QD is quantity demanded, and QS is quantity supplied.
Task: Solve the following:
The demand function is given as QD = 40 - 2P.
The supply function is given as QS = 2P.
Calculate the equilibrium price and quantity. Please provide a step-by-step calculation to show how you arrived at the answer.