Robin mixes two liquids, one blue in colour and other red in colour in the ratio 3:1 and sells the mixture at the rate of $15 per litre, thereby making a 20% profit on his outlay. The blue liquid costs Robin $2 per litre lesser than the red liquid. How much does a litre of red liquid cost?

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Correct answer marked below

$14

Correct

$13

$12

$11

$10

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Explanation

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Official Explanation

Official

In mixture problems, always try to make an equation.

The question says Robin sells 1 litre of mixture for $15. The ratio given is 3:1 for Blue:Red.

Let the multiplier for the ratio be 1 (take small and simple terms which are easy for calculation).

So we have 3 litres of blue liquid and 1 litre of red liquid.

Total mixture = 4 litres.

Robin sells 4 litres of mixture for:

4 \times 15 = \$60

We know blue costs $2 less than red per litre:

B = R - 2

…

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What this question tests

What it tests

Your ability to convert between fractions, ratios, and decimals and to reason proportionally.

Common trap

Cross-multiplying incorrectly or mixing up the ratio parts — a part:part ratio is not a fraction of the whole.