Here's how to calculate the new quantity supplied:

1. Calculate the percentage change in price:

   • Percentage change in price = ((New Price - Old Price) / Old Price) * 100
   • Percentage change¹ in price = ((4 - 5) / 5) * 100 = (-1 / 5) * 100 = -20%

2. Calculate the percentage change in quantity supplied:

   • Percentage change in quantity supplied = Price elasticity of supply * Percentage change in price
   • Percentage change in quantity supplied = 2.5 * (-20%) = -50%

3. Calculate the change in quantity supplied:

   • Change in quantity supplied = (Percentage change in quantity supplied / 100) * Old quantity supplied
   • Change in quantity supplied = (-50 / 100) * 300 = -150 units

4. Calculate the new quantity supplied:

   • New quantity supplied = Old quantity supplied + Change in quantity supplied
   • New quantity supplied = 300 - 150 = 150 units

Therefore, the quantity supplied at a price of Rs 4 per unit is 150 units.