Nutrient boxes

You will write three functions. One gives the residence time of a pool. One gives the net change of the soil phosphate of a field. One gives the years until a pool is used up.

The idea. The residence time is the pool divided by the flow out of it each year. The net change of a pool is what comes in minus what goes out. A pool that loses a fixed amount each year is used up in the pool divided by the loss.

Your tasks.

1. Finish residence_time. It gives the years.
2. Finish net_change. It gives the units a year, from the weathering, the returned phosphate, the uptake and the runoff.
3. Finish years_until_empty. It gives the years until a pool is used up, or None if the pool does not fall.