The formula for computing price elasticity of demand is :

The correct answer is option (3) : $\left(\frac{ΔQ}{Q}\right)×\left(\frac{P}{ΔP}\right)$

Demands for some goods are very responsive to price changes while demands for certain others are not so responsive to price changes. Price elasticity of demand is a measure of the responsiveness of the demand for a good to changes in its price. Price elasticity of demand for a good is defined as the percentage change in demand for the good divided by the percentage change in its price. 

Price elasticity of demand for a good is |eD| = percentage change in demand for the good/ percentage change in the price of the good

     = [(ΔQ/Q)*100]/[(ΔP/P)*100]

     = $\left(\frac{ΔQ}{Q}\right)×\left(\frac{P}{ΔP}\right)$