Often, when per cents are given, they'll be easier to deal with if converted to fractions. In this case, since the dealer is making 25% profit, we can just think of that as 5/4 of the purchase price. A 10% off sale can be thought of as a "90% on" sale, which means in fraction form it's 9/10 of the selling price, which is 5/4 of the purchase price, so we should multiply these two fractions:
5x94x10
giving us 45/40 of the purchase price. Since this is 5/40ths over the purchase price, we can convert 5/40 into 12.5%. (The easiest way to do the arithmetic is to make the denominator into 100 by multiplying both the numerator and denominator by 2.5, giving us 12.5/100, which is 12.5%) Thus the answer is B.
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