The initial volume is 40 liters.
The volume lost on first day is,
[tex]40\cdot\frac{0.5}{100}=40\cdot0.005[/tex]The volume left after first day leak is,
[tex]\begin{gathered} 40-40\cdot0.005=40(1-0.005) \\ =40(0.995) \end{gathered}[/tex]The volume leak on second day is,
[tex]40(0.995)\cdot\frac{0.5}{100}=40(0.995)\cdot0.005[/tex]The volume left after second day leak is,
[tex]\begin{gathered} 40(0.995)-40(0.995)(0.005)=40(0.995)(1-0.005) \\ =40(0.995)\cdot(0.995) \\ =40\cdot(0.995)^2 \end{gathered}[/tex]In same manner if volume decreases for seven days, then volume of gas is,
[tex]40\cdot(0.995)^7[/tex]Answer: Option D
