I have this question:

Water is pouring into a container at a rate given by ri(t)=t^3(gallons/hour) and water is pouring out of the same container at a rate given by ro(t)=e^t(gallons/hour) If there are 100 gallons of water at time t= 0, how many gallons of water will be in the container at t = 4 hours?

Now I'm normally pretty good with this sort of question and I can figure out r and then put in 4 for t, but I'm having a lot of trouble figuring out r because when I put t=0 into either eqaution with gallons=100 I end up with everything equaling 0. :/ Any help? Thanks!