So let's just say w of t, so that's the amount of water in the pipe at any given time t, it's going to be equal to, well we start with 30 cubic feet of water in the pipe at t equals zero, they tell us that right up here. So we start with that much, and then we are going to add or take away some amount. So then it's going to be plus, we're going to sum up from time zero to time t, remember this is a function of t. Well let's sum up our net inflow times little small changes in time, and we'll sum up all those small changes in time from zero to t, and that'll give us our aggregate inflow, our aggregate net inflow. So what is our net inflow? Well you have the function r, which says the rate at which the water is entering, and your function d, which is the rate at which water is exiting. And instead of using t as our variables here, I'm going to use x, since t is already one of our variable, is one of our boundaries of integration. And we can use any variable we want, it's really just a bit of a placeholder variable for the integration.