Here is a nice problem, not too difficult, pertinent to a current hot topic.
Hypothetical scenario: Nothing is adding carbon dioxide to the atmosphere. Carbon dioxide has a 100-year half life in the atmosphere. We crank up a contraption that pumps 100 million tons of carbon dioxide into the atmosphere each year. How much carbon dioxide is in the atmosphere when a steady state is obtained?
Post your answer as a comment below. Extra points for describing the calculation.