A shopkeeper has 15 litres of soft drink “Dew” priced at ₹10 per litre. He wants to add another soft drink “Pepsi” priced at ₹6 per litre.\nHow many litres of Pepsi should be added so that the final mixture costs ₹9 per litre (average price of the mixture becomes ₹9 per litre)?

Introduction:
This is a mixture-cost alligation/weighted average problem. When two liquids with different per-litre prices are mixed, the average price depends on the amounts of each. Since ₹9 per litre is closer to ₹10 than to ₹6, the mixture must contain more of the ₹10 drink than the ₹6 drink.

Given Data / Assumptions:

• Dew quantity = 15 litres at ₹10 per litre
• Pepsi price = ₹6 per litre
• Target mixture price = ₹9 per litre
• Let Pepsi quantity added = x litres

Concept / Approach:
Use the weighted average condition:\n(total cost) / (total volume) = target price.\nSo: (15*10 + x*6) / (15 + x) = 9.

Step-by-Step Solution:
Total cost = 15 * 10 + x * 6 = 150 + 6xTotal volume = 15 + xAverage price condition: (150 + 6x) / (15 + x) = 9150 + 6x = 9(15 + x) = 135 + 9x150 - 135 = 9x - 6x15 = 3xx = 5 litres

Verification / Alternative Check:
If 5 litres of ₹6 drink is added:\nTotal cost = 150 + 5*6 = 180.\nTotal volume = 15 + 5 = 20.\nAverage price = 180/20 = ₹9 per litre, correct.

Why Other Options Are Wrong:
8 litres or 10 litres: adds too much cheaper drink, average would fall below ₹9.12 litres: average drops even further below ₹9.None of these: incorrect because 5 litres works exactly.

Common Pitfalls:
Averaging 10 and 6 directly instead of using quantities.Setting up equation with cost and volume mismatched.Forgetting that the target must lie between the two prices.

Final Answer:
5 litres