In 72 liter mixture of milk and water find the quantity of water if shopkeeper is making a profit percent of 15% after selling at Rs.51.75 /litre and rate of pure milk is Rs.50/liter.

Selling price Rs.51.75/L with 15% profit means cost price = 51.75/1.15 = Rs.45/L. For 72L mixture: total cost = 72×45 = Rs.3240. Pure milk costs Rs.50/L. If milk quantity is m liters and water is w liters: m + w = 72, and 50m + 0w = 3240. So 50m = 3240, giving m = 64.8L and w = 7.2L. Since 7.2L is not in options, 'None of these' is correct.