If elasticity of demand is 0.5, and the price is lowered from $20 to $19, by what percentage will quantity demanded rise?

To find the percentage of demand rise, the formula is

Elasticity = (percentage change of demand/percentage change of price)

Elasticity is equal to 0.5. The percentage change of price is:

Percentage change of rise = (20-19) / 20 * 100 = 5%

Then,

0.5 = Percentage change in demand/5%

Percentage change in demand = 2.5%