Alright, so we know the rate, the rate that things flow into the rainwater pipe. In fact, we could, let me draw a little rainwater pipe here just so we can visualize what's going on. So if this is, if that is the pipe right over there, things are flowing in at a rate of r of t, and things are flowing out at a rate of d of t, and they even tell us that there's 30 cubic feet of water right in the beginning. But these are the rates of entering and the rates of exiting. So how much water, they're asking how many cubic feet of water flow into, so enter into the pipe during the 8 hour time interval. So if you have your rate, this is the rate at which things are flowing into it, they give it in cubic feet per hour. If you multiply it times some change in time, even infinitesimally small change in time, so d t, this is the amount that flows in over that very small change in time.