A mixture contains alcohol and water in the ratio 4 : 3. If 5 litres of water is added to the mixture, the ratio becomes 4 : 5. The quantity of alcohol in the given mixture is -

Answer: Option D

Let the quantity of alcohol and water be 4x litres and 3x litres respectively
$$\begin{array}{rl}& \frac{4x}{\left(3x+5\right)}=\frac{4}{5}\\ & \Rightarrow 20x=4\left(3x+5\right)\\ & \Rightarrow 8x=20\\ & \Rightarrow x=2.5\end{array}$$
Quantity of alcohol
= (4 × 2.5) litres
= 10 litres.