Harish has 36 litres of a 30% acid solution. How many litres of pure acid must he add to raise the strength to 40%?

3.6 litresCorrect answerYour answer4 litresCorrect answerYour answer6 litresCorrect answerYour answer12 litresCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer C

Correct answer: C

Pure acid is a 100% solution.
Solution : pure acid = $(100 - 40) : (40 - 30) = 60 : 10 = 6 : 1$ .
Pure acid = $\frac{36}{6} = 6$ litres.

Worth noting

Adding pure acid also increases the total volume. 3.6 litres simply takes 10% of 36; 4 litres makes the added acid 10% of the final volume, forgetting that the old acid already counts toward the 40%. Check: $(10.8 + 6) \div 42 = 40\%$ .

Concept: Alligation with a pure (100%) component
Typical timeTypical: 90 s
Your timeYours: —

The rule this testsThe alligation rule

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Ten questions from Mixtures & Alligation, at your own pace.

Harish has 36 litres of a 30% acid solution. How many litres of pure acid must he add to raise the strength to 40%?

Explanation