When performing division, the final answer should have the same number of significant figures as the number with the smallest number of sig figs. In this case, both of the given numbers have 3 sig figs, meaning the final answer should have this many as well.
Add all of the pressures, 55, 90, and 50, and divide 100 by the answer you get (195). You'll get 0.512820513 and multiply it by .55 (atm of Oxygen) and you'll get 28.2
