Grain lost in a store

A store holds $100$ tonnes of grain. An untreated store loses $8\%$ of the grain that is left each month. A store that is cleaned, dried and fumigated loses $1\%$ each month. You will follow the untreated store for six months, find when it has lost more than $30$ tonnes, and find how much grain the treatment saves.

The idea. Each month the grain is multiplied by the share that is not lost.

Your tasks.

1. Set untreated to the list of tonnes left in the untreated store at the end of each of the six months, each rounded to 2 decimal places.
2. Set first_bad to the first month in which less than $70$ tonnes are left.
3. Set saved to the tonnes saved by the treatment after six months, rounded to 2 decimal places. Use the last number of untreated for the untreated store.