The ten per cent ladder

You will write functions for the energy at a trophic level, for the number of levels that a chain can have, and for the share that passes between two levels.

The idea. The energy at level $n$ is the energy at the first level times the share, $n - 1$ times. A level can exist only if it gets at least a needed amount of energy.

Your tasks.

1. Finish energy_at. It gives the energy at a trophic level.
2. Finish levels_possible. It gives the number of levels that get at least the needed energy.
3. Finish percent_passed. It gives the share of the lower level that reaches the upper level, as a per cent.