A trader had 1200 kgs of rice. He sold a part of it at 5% profit and the rest at 11% profit, so that he made a total profit of 7%. How much (in kg) rice did he sell at 5% profit?
Let X be the weight of rice sold at 5% profit, then
$$\Rightarrow \frac{105}{100}\times X + \frac{111}{100}\times (1200-x) = \frac{107}{100} \times 1200$$
$$\Rightarrow 1.05\times X + 1.11\times (1200-X) = 1.07\times 1200$$
$$\Rightarrow 1.05\times X - 1.11\times X = 1.07\times 1200 - 1.11\times 1200$$
$$\Rightarrow -0.6\times X = -480$$
$$\Rightarrow X = 800$$
so the answer is option D.
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