Economic theory tells us that in an open market with perfect capital mobility, the equilibrium price is given by the interaction between supply and demand for a good. This point of equilibrium occurs when the supply and demand curves intersect.
In the exercise, initially the supply and demand curves intersect where the price is $ 4 and the quantity demanded is 600. If the cost of production has decreased, the producer can lower the price. If price decreases, demand usually increases, creating a new break-even point. In the case, the price has decreased to $ 3 and demand has increased to 800, which is our answer.