In mixture problem always try to make an equation,

Questions says Robin sells the 1 litre of mixture for $15.

The ratio given is 3:1 for Blue: Red.

Let the multiplier for ratio be 1--(take small and simple terms which is easy for calculation or multiple of 10.

So we have 3 litre of blue liquid and 1 litre of red liquid. So total of 4 litre,

Robin sells 4 litre of mixture for=4* 15= $60

We know that Blue cost $2 less than the red per litre.

So B= R-2 per litre.

here we have 3 litre of blue= \(3(R-2)\)

Now since the question says he gets 20% profit. The equation will be,

\([3(R-2)+ R] + \frac{20}{100} [3(R-2)+ R]= 60.\)

On simplifying,

\(1.2[3R-6+R]= 60\)

\(1.2(4R-6)=60\)

\(4R- 6= 50\).

\(4R= 56\)

\(R= 56/4= 14\)

The answer is A. Hope it helps

Please see the below links for theory.

1.

Work Problem Made Easy2.

Mixture Problems Made Easy3.

Word Problem Made Easy
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