Number of exhaled molecules
Total mass of the atmosphere is equivalent to that in a layer 7 kilometer deep with density equal to the density at the surface. Assuming that air is uniformly mixed, how many molecules of air you have exhaled in the last 20 years will be inhaled in a single breath?

Given:
Earth's surface area=5.11*10^9 square kilometers
Volume at a layer 7 kilometer deep=3.58*10^19 m^3
Volume of air 1 breath equates to 500 cm^3
Volume breathed in 20 years=1.05*10^5 m^3
Assume normal conditions (25 degrees C, 1atm, 24L/mole)
Avogadro number 6.02E23 molecules/mole