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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 inter-changed the number of black and brown pairs by mistake which increased the bill by 100% what was the number of pair of brown socks in the original order?

Correct answer: D. 25

  • A. 10
  • B. 15
  • C. 20
  • D. 25

Explanation

Let the price of one brown pair be p and let the number of brown pairs be n, so one black pair costs 3p. The original bill is 5 x 3p + np, while the mistaken bill is 5p + 3np; a 100% increase means 5p + 3np = 2(15p + np), giving n = 25 pairs. The likely distractor is 20, which comes from setting the increase equal to the original bill incorrectly.

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