Rate of volume change where R is some constant. Since Rate of change is constant it wil have a constant 'gradient' if graphed, and thus we can do rise over run:
Thus by integrating
at and from before at
Can you do the next one on your own? I'll post the answer in 15 minutes.
Edit: Actually after looking at it the second part is ridiculously easy so I don't think you need help with that :)
Thank you very much behemoth100 and CaptainBlack :D . And yes don't worry behemoth100 and I can work out ii) now :)