The fatigue timer

A simple model says that one contraction makes lactic acid in proportion to the share of the ATP that is made without oxygen. If all of the ATP is made without oxygen, one contraction makes 1 unit of acid. A fibre is tired when it has made 40 units. You will write the model and use it.

The idea. The more ATP is made with oxygen, the less acid one contraction makes, and the more contractions the fibre can make.

Your tasks.

1. Finish acid_made. It gives the units of acid after a number of contractions, for a share of the ATP made with oxygen.
2. Finish contractions_before_fatigue. It gives the number of contractions before the fibre is tired.
3. Finish fibre_kind. It gives "red" if 50 per cent or more of the ATP is made with oxygen, and "white" if less.