Answer
$5\times10^{2} $ mL
Work Step by Step
If inhaled air contains 20% oxygen and 16% is exhaled, the pure oxygen needed must then come from the 4% difference. So, the 240 mL/min of pure oxygen needed is 4% of the volume of inhaled air.
From this information, we could calculate the volume of inhaled air per minute.
$ Vol = \frac{240 \frac{mL}{min}}{0.04} = 6000 \frac{mL}{min}$
If a person breathes 12 times per minute, the volume of air per breath must be:
$6000\frac{mL}{min} \times \frac{1 min}{12 breaths} = \frac{500 mL}{breath} = 5\times10^{2} mL $
