Part C

Input := 


solC = DSolve[{vol2'[t]==-k2*vol2[t],vol2[0]==300},vol2[t],t]

Output =


              300

{{vol2[t] -> -----}}

              k2 t

             E

Input := 


vol2[t_]=vol2[t]/.solC[[1]]

Output =


 300

-----

 k2 t

E

This is an exponential decay function. Now find k2.

We seek the time it takes to go from 300 ml to 150 ml. That is apparently t = 1 min.