A milkman buys pure milk at ₹25 per litre. He adds water equal to 1/4 of the milk quantity (i.e., 25% of the milk volume) and sells the mixture at ₹26 per litre.\nWhat is his gain percentage on the total cost price?

Introduction:
This question tests profit percentage when water is added for free and the mixture is sold at a slightly higher rate. The trick is to compare cost (only milk) versus revenue (milk + free water). A convenient method is to assume a simple milk quantity that matches the given fraction (like 4 litres), so that 1/4 water becomes 1 litre.

Given Data / Assumptions:

• Milk cost price = ₹25 per litre
• Water added = 1/4 of milk quantity
• Selling price of mixture = ₹26 per litre
• Water cost assumed = ₹0 (not given otherwise)

Concept / Approach:
Assume milk quantity = 4 litres to match the 1/4 fraction easily.\nThen water added = 1 litre, mixture volume = 5 litres.\nCompute total cost, total revenue, profit, then profit%.

Step-by-Step Solution:
Assume milk bought = 4 litresWater added = (1/4) * 4 = 1 litreTotal mixture volume sold = 4 + 1 = 5 litresTotal cost = 4 * 25 = 100Total revenue = 5 * 26 = 130Profit = 130 - 100 = 30Profit percentage = (30 / 100) * 100 = 30%

Verification / Alternative Check:
Effective cost per litre of mixture = 100/5 = ₹20.\nSelling at ₹26 means profit per litre = ₹6.\nProfit% = 6/20 * 100 = 30%, confirming the same result quickly.

Why Other Options Are Wrong:
25%: often comes from comparing 26 to 25 directly, ignoring free water.20% or 15%: happens when using wrong total litres or wrong base for profit%.10%: drastically underestimates because volume increase is ignored.

Common Pitfalls:
Calculating profit% using (26-25)/25 instead of mixture economics.Forgetting that 1/4 water is based on milk quantity, not final mixture.Taking profit% on selling price rather than on cost price.

Final Answer:
30%