A mobile retailer sells three smartphones in a day—Brand A, Brand B, and Brand C. The total cost price of the three phones is Rs. 80,000. The retailer earns profit (on cost price) at the following rates: Brand A: 12%, Brand B: 6% and Brand C: 9%. The cost price of Brand B phone is half the cost price of the Brand C phone. If the total profit for the day is Rs. 7,200, then the profit earned from the Brand B phone (in Rs.) is:

Let the cost price of Brand B be x, so the cost price of Brand C is 2x, making the cost price of Brand A 80000 minus 3x. Using the profit rates, the total profit equation is 12% of 80000 minus 3x plus 6% of x plus 9% of 2x equals 7200. This simplifies to 9600 minus 0.36x plus 0.06x plus 0.18x equals 7200, which gives 0.12x equals 2400, so x equals 20000. The profit earned from Brand B is 6% of its Rs. 20000 cost price, which equals Rs. 1200.

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