The sum of the partial pressures of all of the atmospheric gasses is equal to the total air pressure at any given elevation on Earth. It is important to note that the percent composition of atmospheric gasses is the same at all elevations on Earth, it is simply the air pressure of these gasses that decreases as elevation increases. So to find the percent composition of oxygen in the atmosphere:
(0.20946 atm O2/1 atm air)*100=20.946% O2 in the atmosphere
Since the total air pressure at the top of Mount Everest is 253 torr we can multiply this by the percent of O2 in the atmosphere to get our answer:
253 torr*0.20946=52.99 torr is the partial pressure of oxygen at the top of Mount Everest.