Leaves the quantity demanded unchanged. Results in a shortage of 20 units. Results in a surplus of 10 units. Results in a shortage of 40 units. According to the invisible hand principle, competitive markets generally bring the self-interest of individuals into harmony with the efficient allocation of resources even though centralized planning of economic activities is absent.
Market prices communicate information to buyers and sellers.
Market prices coordinate the decisions of buyers and sellers. Market prices indicate to policy makers those goods on which price controls should be imposed. Market prices motivate entrepreneurs to produce those products that are currently most desired relative to their costs of production. If an economic action generates more costs than benefits, then the action by definition increases the nation's economic welfare. The flu shot is cheaper than the cost of treatment when you get the flu.