A solution containing alcohol and water in the ratio 7 : 2 is mixed with twice the amount of another solution containing alcohol and water in the ratio 5 : 1. What is the percentage of alcohol in the final mixture?

Let 9 litres of the first solution be mixed with 18 litres of the second solution.
Therefore, total volume of the solution = (9 + 18) litres = 27 litres
Quantity of alcohol in the first solution = $$\dfrac{7}{9} * 9$$ litres = 7 litres
Quantity of alcohol in the second solution = $$\dfrac{5}{6} * 18$$ litres = 15 litres
Total quantity of alcohol = (7 + 15) litres = 22 litres
% of milk = $$\dfrac{22}{27} * 100\% = 81.48\%$$
Hence, option E is the correct answer.