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More specific, I found out that the amount of oxygen we inspire is 21% from the total amount, with 0.04% CO2. The expired amount is 16% oxygen, with 4% CO2. Where does the remaining 1% oxygen go? Or are these only approximations?

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When you write the number "21%", it represents equally all numbers rounded to 21%; that is, 21% is the same number as 20.5% and 21.49% under the most common rounding convention.

When you subtract two rounded numbers, you can easily get a number that is not zero. For example, 10.500 - 10.499 = 0.001 before you round, but if you round first, 11 - 10 = 1. 10.501 - 10.500 has the same difference, but if you round first and subtract you'll get 0. In other words, 16 + 4 = 21 is completely reasonable math in the world of rounding, the value of "1" is not meaningfully different from zero on that scale.

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  • $\begingroup$ Got it. So it was just an approximation. Thanks! $\endgroup$
    – me1234
    Jan 2 at 21:23
  • $\begingroup$ @me1234 I wouldn't call it an "approximation" per se; it could be that the numbers were measured very very precisely, but if they've been reported to you as a number like "16%" you should know by looking at that number that it's clearly been rounded to an integer percentage value. $\endgroup$
    – Bryan Krause
    Jan 2 at 21:23

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