A trader had 960 kgs of rice. He sold a part of it at 20% profit and the rest at 8% profit, so that he made a total profit of 12%. How much rice (in kgs) did he sell at 8% profit?
Let X be the weight of wheat sold at 8% profit, then
$$\Rightarrow \frac{108}{100}\times X + \frac{120}{100}\times (960-x) = \frac{112}{100} \times 960$$
$$\Rightarrow 1.08\times X + 1.2\times (960-X) = 1.12\times 960$$
$$\Rightarrow 1.08\times X - 1.2\times X = 1.12\times 960 - 1.2\times 960$$
$$\Rightarrow -0.12\times X = -76.8$$
$$\Rightarrow X = 640$$
so the answer is option D.
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