Reading a chromatogram

A student ran a chromatogram of a leaf extract and measured how far each pigment moved. You will find the Rf value of each, name the pigment that travelled the farthest, and sort the pigments from the farthest to the least.

The idea. The distances are in a dictionary, in centimetres. The solvent front moved $10\ \text{cm}$.

Your tasks.

1. In rf, return the Rf value of a pigment, its distance divided by the solvent distance.
2. Set farthest to the name of the pigment with the highest Rf.
3. Set order to a list of the pigment names from the highest Rf to the lowest.