A shopkeeper has 80 litres of milk. He makes a profit of 25% by selling 1 litre of milk at Rs.15. How much water should he mix in the milk so as to increase his profit to 30% if he plans to sell the milk-water mixture at Rs. 15 per litre??

Sure, let's solve this step by step.

Step 1: Calculate the cost price of 1 litre of milk The shopkeeper makes a profit of 25% by selling 1 litre of milk at Rs.15. So, the cost price (CP) of 1 litre of milk can be calculated as follows: CP = Selling Price (SP) - Profit Since Profit = 25% of CP and SP = Rs.15, we can write the equation as: CP = 15 - 0.25*CP Solving this equation, we get CP = Rs.12

Step 2: Calculate the new selling price per litre to get a profit of 30% The shopkeeper wants to make a profit of 30% by selling the milk-water mixture at Rs.15 per litre. So, the new cost price (new CP) can be calculated as follows: new CP = SP - Profit Since Profit = 30% of new CP and SP = Rs.15, we can write the equation as: new CP = 15 - 0.30*new CP Solving this equation, we get new CP = Rs.11.54 (approx)

Step 3: Calculate the amount of water to be mixed The shopkeeper initially has 80 litres of milk which costs him Rs.12 per litre. So, the total cost of milk = 80*12 = Rs.960 Now, he wants to sell the milk-water mixture at a cost price of Rs.11.54 per litre. So, the total quantity of the mixture that he can sell with the same amount of money = 960/11.54 = 83.19 litres (approx) Therefore, the amount of water that he should mix with the milk = 83.19 - 80 = 3.19 litres (approx)

So, the shopkeeper should mix approximately 3.19 litres of water in the milk to increase his profit to 30% if he plans to sell the milk-water mixture at Rs. 15 per litre.