A can holds 20 litres of oil. Each time, 2 litres are drawn off and replaced with a cheaper oil. After how many such replacements are 16.2 litres of the original oil left?

1

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2

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3

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CorrectNot quiteAnswer B

Correct answer: B

$20\left(\frac{9}{10}\right)^{n} = 16.2$ , so $\left(\frac{9}{10}\right)^{n} = 0.81$ .
$0.81 = 0.9^2$ , so $n = 2$ .

Worth noting

Removing 2 litres twice would leave 16; the second draw takes out only 1.8 litres of the original oil.

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

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

Explanation