How much apple juice, at Rs. 5 per litre, should be added to 12 litres of mango juice, at Rs. 10 per litre, to make the price of the mixed juice Rs. 8 per litre?

Using the rule of alligation, the cost price of apple juice at 5 rupees and mango juice at 10 rupees are mixed to achieve a price of 8 rupees. The ratio of the quantities is the difference between the higher price and the mean price to the difference between the mean price and the lower price. This gives a ratio of mango to apple juice of 3 to 2; since there are 12 litres of mango juice, the calculation yields 8 litres of apple juice.

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