A milkman buys milk at ₹60 per litre. How much water should he add per litre so that by selling the mixture at ₹60 per litre he makes a 20% profit?

Water costs nothing, so mixing it in lowers the cost per litre of what he sells. He keeps the price at ₹60, and the profit comes entirely from the cheaper mixture. First find what each litre of mixture must cost him, then use alligation between milk at ₹60 and water at ₹0.

Question: Milk costs ₹60/L. Selling the mixture at ₹60/L should give 20% profit.

1. Required cost of mixture = 60 / 1.2 = ₹50 per litre.
2. Alligation between milk (₹60) and water (₹0), mean ₹50:

• Milk : water = (50 - 0) : (60 - 50) = 50 : 10 = 5 : 1.

1. So add 1/5 litre = 200 ml of water per litre of milk.

Check: 1.2 L of mixture costs ₹60 (only the milk costs money) and sells for 1.2 x 60 = ₹72. Profit ₹12 on ₹60 = 20%.

const cost = 60 / 1.2;
console.log(cost); // Output: 50
console.log(((1.2 * 60 - 60) / 60) * 100); // Output: 20

• Thinking 20% profit means the mixture is 20% water; here water is 0.2 L in 1.2 L, which is 1/6 of the mixture.
• Taking 20% of ₹60 off the cost and getting ₹48 instead of dividing by 1.2.