Exhaled air contains 16% oxygen by volume. Assuming you exhale like the average person does during strenuous exercise (that is, you exhale 1L of air per breath and you exhale 40 breaths per minute) ad assuming the dive will take 90 minutes, what volume of oxygen will you end up exhaling?

Exhaled air for 90 minutes would be 40 breaths per minute (at 1 litre per breath) x 90 minutes = 3600 litres x 0.16 = 576 litres of oxygen, In other words, first work out the amount of air breathed out in litres/minute and then multipy by the number of minutes to get the total litres of air breathed out and then multiply that figure by 0.16 (16 % oxygen per volume of air) to get total litres of oxygen breathed out.