How many kilograms of salt, at Rs. 28 per kg must be mixed with 39.6 kgs of salt, at Rs. 16 per kg, so that by selling the mixture at Rs. 29.90, there is a gain of 15%?

Selling price with 15% profit means cost price = 29.90/1.15 = Rs. 26 per kg. Using the mixture formula: (28 - 26)/(26 - 16) = 2/10 = 1/5. This means the quantities are in the ratio 1:5. Since the quantity of Rs. 16 per kg salt is 39.6 kg, the quantity of Rs. 28 per kg salt = 39.6/5 = 7.92 kg. Checking: (7.92 × 28 + 39.6 × 16)/47.52 = 26, which is correct. Multiplying by 25 gives 198 kg.