$a)40l$

 

$b)20l$

$c)po6min$

$d)V\left(t\right)=at+b,a,b=?$  

$\{\begin{array}{l}V\left(0\right)=40\\ V\left(8\right)=0\end{array}$

$\{\begin{array}{l}a\cdot 0+b=40\\ a\cdot 8+b=0\end{array}$

$\{\begin{array}{l}b=40\\ 8a+40=0\mid -40\end{array}$

$\{\begin{array}{l}b=40\\ 8a=-40\mid :8\end{array}$

$\{\begin{array}{l}b=40\\ a=-5\end{array}$

$V\left(t\right)=-5t+40,t\in ⟨0;8⟩$