The supply function for a good is $q = -20 + 4p$. Using the point method, the elasticity of supply when price is Rs 10 equals:

1. Point elasticity: $E_s = \dfrac{dq}{dp} \times \dfrac{p}{q}$. 2. Differentiate: $\dfrac{dq}{dp} = 4$. 3. At $p = 10$: $q = -20 + 4(10) = -20 + 40 = 20$. 4. $E_s = 4 \times \dfrac{10}{20} = 4 \times 0.5 = 2.0$. Using $dq/dp$ alone wrongly gives 4. _Source: ICAI BoS CA Foundation Paper 4 Business Economics, Ch 2 Unit III "Supply", p.9_