What is the molarity of CO2 in the water that is saturated with air

The aqueous solubility at 20 degrees C of CO2 at 1 atm is equivalent to 878 mL CO2 gas measured at STP (1atm 0 degree C) per liter of water. What is the molarity of CO2 in the water that is saturated with air at 1atm and 20 degrees C? air contains o.o397% CO2 by mol. I need to see they way this is figured out in order to learn. So answer and calculations.