CAT 2025 Slot 1 QA Q9 Ratio, Mixtures & Averages

Question 9 of the Quantitative Ability (QA) section, CAT 2025 Slot 1, with CAT's official answer and a worked solution.

A container holds 200 litres of a solution of acid and water, having 30% acid by volume. Atul replaces 20% of this solution with water, then replaces 10% of the resulting solution with acid, and finally replaces 15% of the solution thus obtained, with water. The percentage of acid by volume in the final solution obtained after these three replacements, is nearest to
  1. A
    23
  2. B
    25
  3. C
    29
  4. D
    27

Answer: (D) 27 Ratio, Mixtures & Averages MCQ

Solution

  1. Start: \(30\%\) of 200 = 60 L acid.
  2. Replacing 20% with water leaves \(60 \times 0.8 = 48\) L acid.
  3. Replacing 10% with acid: \(48 \times 0.9 + 20 = 63.2\) L.
  4. Replacing 15% with water: \(63.2 \times 0.85 = 53.72\) L, which is \(\frac{53.72}{200} \approx 26.9\%\), nearest to 27.

Same concept in other years

See every question on Repeated replacement of a mixture: concentration × (1 − f) each time.

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