Could you give us a source for it?
If you Google ‘composition of air’, you’ll find lots of links where they show the various constituents of ‘dry air’. Here’s one - http://mistupid.com/chemistry/aircomp.htm
It’s a bit more fuzzy to determine the average value of water vapor in the atmosphere. There is a LOT of difference from hot to cold and rainforest to desert. I read somewhere that the average is 2% ... which seems like a reasonable value. I’ve read elsewhere that it ranges from 1-4%.
Anyway, assume 2% is H2O, adjust to 100% and round off your numbers ... done.
I believe the best part of the discussion is to put the relative percentages in terms of dollars and cents. A junior high schooler might get lost in percentages or fractions or ppm. But I guarantee you they understand the difference between the $20. worth of O2 (animals breathe this) and 4 cents worth of CO2 (plants breathe this).
It’s a simple way to change the histerical “OMG we’ve added 100ppm of CO2” into “oh, yea, all of our emissions put together adds up to a single penny of CO2, it really is neglible”. And, BTW, if we’d subtracted a penny’s worth of CO2 instead of adding it - the plants would be dying now...