My daughter is studying biochemistry at undergraduate level. This is the question we're stuck on from her text book.

Suppose that you are climbing a high mountain and the oxygen partial pressure in the air is reduced to 75 torr. Estimate the percentage of the oxygen carrying capacity that will be utilized, assuming that the pH of both tissues and lungs is 7.4 and that the oxygen concentration in the tissues is 20 torr.

The answer is 62.7% - that's at the back of the book. But we're trying to figure out exactly how that is calculated. A 7.4pH indicates that the pH is normal, so no need to adjust for that.

Can the answer be calculated from the 20 torr partial pressure in the tissues, and the 75 torr partial pressure in the air? If not, what other information do I need? Maybe a standard conversion or assumption - like I'm starting the climb at sea level? (I can't work out how to do it from the text before the exercises

).