r of t minus d of t dt, so this is the amount of total water in the pipe at time w. Well, this is going to be equal to 50. So we have just written an equation involving one or more integrals that gives the time w when the pipe will begin to overflow. So if you could solve for w, that's the time that the pipe begins to overflow. And we are assuming that it doesn't just right get to 50 and then somehow come back down, that it will cross 50 at this time right here. And you could test that a little bit more if you want. You could try the slightly larger w, or you could see that the rate that you have more flowing in than flowing out at that time, so this r of w is going to be greater than d of w, so it means you're only going to be increasing, so you're going to cross over right at that time. So if you wanted that, want the w though, you'd solve this.