when price of a commodity reduces from Rs 5 per unit to Rs 4 per unit , expenditure on the commodity reduces from Rs 60 to Rs 40 . find price elasticity of demand .

Given:
Initial Expenditure (E₀) = Rs 60
Initial Price (P₀) = Rs 5
Final Expenditure (E₁) = Rs 40
Final Price (P₁) = Rs 4
Elasticity of demand = ?

$$\begin{gathered} E_0 =P_0\times Q_0 \\ \mathrm{i}.\mathrm{e}. 60 = 5\times Q_0 \\ \mathrm{so}, Q_0=12 \\ {E}_1 =P_1\times Q_1 \\ \mathrm{i}.\mathrm{e}. 40 = 4\times Q_1 \\ \mathrm{so}, Q_1=10 \\ \mathrm{Change} \mathrm{in} \mathrm{quantity} \mathrm{demanded} \left(\Delta Q\right)=Q_1-Q_0=-2 \\ \mathrm{Change} \mathrm{in} \mathrm{price} \left(\Delta P\right)=P_1-P_0=-1 \\ \mathrm{Price} \mathrm{elasticity} \mathrm{of} \mathrm{demand} =\frac{\mathrm{\Delta }Q}{\mathrm{\Delta }P}\times \frac{P}{Q}=\frac{-2}{-1}\times \frac{5}{12}=0.83 \end{gathered}$$

Note- I think there is something wrong with the question, since elasticity of demand is coming out to be positive. Please cross-check. If everything is fine, then this is the case of Giffen good, since as price falls, quantity demanded also falls.