A new mine has tons of coals in reserve. Every year, it plans to sell tons.
Define a function to model the amount of coals, and locate the points to answer the following questions:
  1. After years, there will be tons of coals left.
  2. After years, there will be tons of coals left.
  3. At this rate, coals in the mine will be used up in years.

You can earn partial credit on this problem.