Five litres of water is added to a certain quantity of pure milk costing Rs. 3 per litre. If by selling the mixture at the same price as before, a profit of 20% is made, what is the amount of pure milk in the mixture?

Let pure milk = x litres. Cost = Rs. 3/litre, so total cost = 3x. After adding 5 litres water, total volume = (x+5) litres. Selling at Rs. 3/litre gives revenue = 3(x+5) = 3x + 15. Profit = 20% of cost = 0.2 × 3x = 0.6x. So 3x + 15 = 3x + 0.6x + 3x = 4.2x. This gives 15 = 1.2x, so x = 12.5. Wait, let me recalculate. Actually: Profit% = (Profit/Cost) × 100. 20 = ((3(x+5) - 3x)/3x) × 100 = (15/3x) × 100 = (5/x) × 100. So 20 = 500/x, x = 25. Answer is 25 litres of pure milk.