Multiple choice

A laboratory tank holds 300 litres of a solution of reagent and water, having 40% reagent by volume. A technician drains 50% of this solution and replaces it with water, then drains 25% of the resulting solution and replaces it with pure reagent. Finally, he drains 25% of the solution thus obtained and replaces it with water. The percentage of reagent by volume in the final solution obtained after these three replacements is:

  1. 26%

  2. 28%

  3. 30%

  4. 32%
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Start: 300L, 120L reagent (40%). 1) Drain 50% (150L left, 60L reagent), add 150L water: 60L reagent. 2) Drain 25% (112.5L left, 45L reagent), add 75L pure reagent: 45+75 = 120L reagent. 3) Drain 25% (225L left, 90L reagent), add 75L water: 90L reagent. Final % = 90/300 = 30%.

AI explanation

The initial amount of reagent is 40 percent of 300, which is 120 litres. Draining 50 percent and replacing it with water leaves 120 times 0.50, which is 60 litres of reagent. Draining 25 percent of the solution and replacing it with pure reagent adds 75 litres, making the total 60 times 0.75 plus 75, which is 120 litres of reagent. Finally, draining 25 percent and replacing it with water leaves 120 times 0.75, which is 90 litres of reagent. The final percentage is 90 divided by 300 times 100, which is 30 percent.

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