Price decreases by 20 percent.

The elasticity is negative, implying that quantity demanded will increase when price decrease. Since we know quantity demanded increases, we therefore know price must have declined.

Suppose price decreases by x percent. By the definition of price elasticity, we know that quantity demand will increase by 1.2*x percent. Since we know that quantity demanded increases by 24%, we must have 1.2*x = 24, or x = 20.