A vessel holds 81 litres of milk. A certain amount is taken out and replaced with water, and this is done twice. 64 litres of milk are left. How much was taken out each time?

8 litresCorrect answerYour answer8.5 litresCorrect answerYour answer9 litresCorrect answerYour answer17 litresCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer C

Correct answer: C

$81\left(1 - \frac{x}{81}\right)^2 = 64$ , so $1 - \frac{x}{81} = \frac{8}{9}$ .
$\frac{x}{81} = \frac{1}{9}$ , so $x = 9$ litres.

Worth noting

8.5 litres splits the 17 litres lost evenly between the two draws; the second draw takes less milk.

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

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

Explanation