The dialysis timer

A one pool model says that the urea left after dialysis is $100\,e^{-r}$ per cent, where $r$ is the volume of blood cleared divided by the body water. The body water is 40 L. You will write the model and find how long a session must be.

The idea. The volume cleared in $h$ hours is the clearance in mL a minute times $60 \times h$ divided by 1000, in litres.

Your tasks.

1. Finish ratio. It gives the volume cleared divided by the body water.
2. Finish urea_left. It gives the per cent of the urea that is left.
3. Finish hours_for_fall. It gives the hours that a clearance needs to remove a given per cent of the urea.