How much pepsi at Rs.6 a litre is added to 15 litre of 'dew' at Rs. 10 a litre so that the price of the mixture be Rs.9 a litre?

Step 1: Understand the problem

• We have 15 litres of Dew costing Rs. 10 per litre.
• We want to mix it with some quantity of Pepsi costing Rs. 6 per litre.
• The final mixture should cost Rs. 9 per litre.
• We need to find how much Pepsi should be added to get the desired mixture price.

Step 2: Let the quantity of Pepsi added be x litres

• Total volume of mixture = 15 litres (Dew) + x litres (Pepsi)
• Total cost of Dew = 15 × 10 = Rs. 150
• Total cost of Pepsi = x × 6 = Rs. 6x
• Total cost of mixture = 150 + 6x
• Average cost of mixture = (150 + 6x) / (15 + x)

Step 3: Set up the equation based on desired average cost

(150 + 6x) / (15 + x) = 9

Step 4: Solve the equation

150 + 6x = 9(15 + x)
150 + 6x = 135 + 9x
150 - 135 = 9x - 6x
15 = 3x
x = 5

Answer: 5 litres

We must add 5 litres of Pepsi to the 15 litres of Dew to get a mixture costing Rs. 9 per litre.

This problem demonstrates the practical application of the weighted average concept. It’s commonly seen in real-world scenarios like mixing ingredients, pricing strategies, or even fuel blending. By using a straightforward algebraic approach, you can determine how to balance quantities of different cost items to achieve a target cost per unit.

Such problems are useful for students preparing for aptitude tests, competitive exams, or those interested in business and trade calculations.