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. So our net inflow is r, I'll say r of x, minus d of x, and then dx. Once again, this is our net rate of inflow, this is our inflow minus our outflow, so our net rate of inflow times small changes, you could view this as time, although it's x, and then we sum up from zero to time is equal to t. So this is how much water we are going to have in the pipe at any time t. So let's now think about at what point does w hit a minimum in this interval? And there's three possibilities where w could hit a minimum point.