The pest counter

A farmer sprays a crop and wants to know how long the larvae will take to fall below the level that the crop can bear. You will write the sums and a rule for deciding to act.

The idea. If a share of the larvae die each day, the number is multiplied by $1 - \text{share}/100$ each day.

Your tasks.

1. Finish larvae_after. It gives the larvae on a plant after some days.
2. Finish days_below. It gives the first whole day on which the larvae are at or below the limit.
3. Finish must_act. It gives True if the average of the counts is above the limit.