Hello everyone! Boy these problems are so interesting, but I'm struggling to get the equation setup correct. Either way I find these problems so tasty.

A water tank can be emptied by using one pump for 5 hours. A second, smaller pump can empty the tank in 8 hours. If the larger pump is started at 1:00 p.m. at what time should the smaller pump be started so that the tank will be emptied at 5:00 p.m. ?

So 5 hours passes.

In one hour the bigger pump empties 1/5 of the tank

In one hour the smaller pump empties 1/8 of the tank

Let x be the amount of hours.

So I thought: $\displaystyle \frac{1}{5}x\,+\,\frac{1}{8}(5\,-\,x)\,=\,\frac{1}{5}$

But that make no sense. Can someone show me how to get the equation?