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UPCAT Mathematics practice question 24: How much pure water must be added to 10 liters of a 30% acid solution to dilute it to 20% acid?

Correct answer

A. 5 liters

The acid stays at 3 L. If x liters of water are added, 3 = 0.20(10 + x), so 10 + x = 15 and x = 5.

How much pure water must be added to 10 liters of a 30% acid solution to dilute it to 20% acid?

  1. 5 litersCorrect
  2. 2 liters
  3. 3 liters
  4. 10 liters

Why A is correct

The acid stays at 3 L. If x liters of water are added, 3 = 0.20(10 + x), so 10 + x = 15 and x = 5.

Why the other options are wrong

B 2 liters
Solves for a 25% target rather than 20%.
C 3 liters
Confuses the 3 litres of acid present with the volume of water to add.
D 10 liters
Assumes halving the concentration means doubling the volume. Going from 30% to 20% is a factor of 1.5, not 2.
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