Multiple choice

A solution of 25 litres contains 80% alcohol. How many litres of water must be added to this mixture so that the concentration of alcohol in the solution becomes 66.67%

  1. 5 litres

  2. 6 litres

  3. 7 litres

  4. 8 litres

  5. 10 litres
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A Correct answer
Explanation

Initial alcohol = 80% of 25 = 20 litres. Let x be the water added. New total volume = 25 + x. We want 20 / (25 + x) = 66.67% = 2/3. Thus, 60 = 50 + 2x, so 2x = 10, x = 5 litres.

AI explanation

Using the formula for replacing part of a mixture, the initial amount of pure alcohol is 80 percent of 25 litres, which is 20 litres. The required concentration of alcohol is 66.67 percent, which is exactly the fraction 2/3, so the final total volume of the solution must satisfy 20 divided by the final volume equals 2/3. Solving this gives a final volume of 30 litres, meaning 30 minus 25, or 5 litres of water, must be added.

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