5l of 70% alcohol solution was mixed with 9l of pure water what is the concentration of the mixture

The new mixture has a total volume of 5 L + 9 L = 14 L.

70% of the 5 L alcohol solution is alcohol, so that it contributed 3.5 L of alcohol to the mixture. The pure water contributed no alcohol. So in the mixture, the concentration of alcohol is

3.5 L / 14 L = 0.25, or 25%