600 litres of a 12% acid solution are in a tank. How many litres of a 30% acid solution must be added so that the mixture is more than 15% but less than 18% acid?

More than 120 and less than 300 litresCorrect answerYour answerMore than 100 and less than 200 litresCorrect answerYour answerMore than 120 and less than 180 litresCorrect answerYour answerMore than 150 and less than 300 litresCorrect answerYour answer

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CorrectNot quiteAnswer A

Correct answer: A

With $x$ litres added, the acid is $72 + 0.3x$ in $600 + x$ litres.
$72 + 0.3x > 0.15(600 + x)$ gives $x > 120$ ; $72 + 0.3x < 0.18(600 + x)$ gives $x < 300$ .

Worth noting

Each limit must count the added litres in the new total, $600 + x$ .

Concept: Two conditions on a mixture
Typical timeTypical: 120 s
Your timeYours: —

The rule this testsInequalities from situations

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600 litres of a 12% acid solution are in a tank. How many litres of a 30% acid solution must be added so that the mixture is more than 15% but less than 18% acid?

Explanation