How many litres of a 30% alcohol solution should be added to 40 litres of a 60% alcohol solution to prepare a… · ABC26GN2474

Let $x$ litres of 30\% alcohol solution be added. Then, $30 \%$ of $x+60 \%$ of $40=50 \%$ of $(x+40)$ $$\begin{aligned} & \Rightarrow \quad 30 x+60 \times 40=50(x+40) \\ & \Rightarrow \quad 30 x+2400=50 x+2000 \\ & \Rightarrow \quad 20 x=400 \\ & \Rightarrow \quad x=20 . \end{aligned}$$