Why small cells win

You will work out the ratio of surface area to volume for cube shaped cells with sides from 1 to 20 micrometres, draw it, and find the first cell whose ratio falls below a limit.

The idea. A cube of side $L$ has a surface area of $6L^2$ and a volume of $L^3$. A cell needs enough surface for its volume.

Your tasks.

1. In ratio, return the surface area divided by the volume.
2. Set ratios to the ratio for every side in sides.
3. Set first_below to the first side whose ratio is below limit.