A person ordered 5 pairs of black socks and some pairs of brown socks. The price of a black pair was thrice that of a brown pair. While preparing the bill, the bill clerk interchanged the number of black and brown pairs by mistake which increased the bill by 100%. What was the number of pairs of brown socks in the original order?
- A 10
- B 15
- C 20
- D 25
Show answer and explanation
Correct answer: D - 25
Let the price of a brown pair be b, so a black pair costs 3b. Let the original number of brown pairs be x. Original bill = 5 black + x brown = 5(3b) + x(b) = 15b + xb. After interchanging the counts, there are x black pairs and 5 brown pairs, so the wrong bill = x(3b) + 5(b) = 3xb + 5b. A 100% increase means the wrong bill is twice the original: 3xb + 5b = 2(15b + xb). Dividing by b: 3x + 5 = 30 + 2x, so x = 25.