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      Question

      The average number of books on Biology, Chemistry, and

      Physics in a book shop is 300. Biology books are 150 more than the average number of Chemistry and Physics books. The number of Physics books is 150 less than the average number of Biology and Chemistry books. What is the number of chemistry books in the shop?
      A 300 Correct Answer Incorrect Answer
      B 360 Correct Answer Incorrect Answer
      C 420 Correct Answer Incorrect Answer
      D 250 Correct Answer Incorrect Answer

      Solution

      Let the number of biology, chemistry and physics books be B, C and P. B+C+P =300×3=900 B+C+P=900      … (1) Biology books =150+Average number of (C and P) B=150+(C+P)/2 2B=300+C+P 2B-C-P=300      … (2) Physics books =Average of (B+C)/2 -150 P=B+C)/2 -150 2P=B+C-300 B+C-2P=300     … (3) Adding Equation (1) and (3) and subtracting (2) ... (B+C+P) + B+C-2P -2B+C+P=900+300-300 3C=900 C=300.

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