Since we want the pressure to remain constant we have to use Charles Law of gases, that relates the volume and the temperature at a constant pressure:
[tex]\frac{V1}{T1}=\frac{V2}{T2}[/tex]In this case, we know the values of V1, T1 and V2, which are 10L, 25°C and 6.0L. Using these values we have to find T2:
[tex]\begin{gathered} T2=\frac{V2\cdot T1}{V1} \\ T2=\frac{6.0L\cdot25}{10L} \\ T2=15 \end{gathered}[/tex]The gas has to be stored at 15°C.