So to calculate the number of molecules exchanged by the lungs every single minute we have to calculate the number of moles of air that is exchanged and to calculate the moles we can use the ideal gas law. So n is equal to where n is our number of moles is equal to the pressure in ATM multiplied by the volume in liters divided by the gas constant r multiplied by the temperature given to us in Kelvins. Now, the pressure we're assuming is one ATM the volume we calculated to be six liters the ideal gas law is 0.20.81 ATM times liter divided by kelvin times mole and the temperature in Kelvin is equal to 21 plus 273 and that gives us about 294 Kelvins. Now, if we multiply and divide we get about zero point 25 moles. Now, this is the number of moles and we know inside 1 mol we have abogandro number of molecules and so to find number of molecules we take this many moles multiplied by avogadro's number 6.2 times ten to 23 molecules per mole the moles cancel and we're left with about 1.51 times ten to the 23 molecules. So this is the number of molecules that are exchanged by our lungs every single minute assuming that the individual is at rest.