What is the variation of atmospheric pressure at large heights?
From the given composition of the atmosphere you can calculate the mean molecular mass $M$ of air.

Since Earth's gravitational field and temperature are assumed to be constant you can use the barometric formula

$$p(h_1) = p(h_0)~\mathrm{e}^{- \frac{M g}{R T} \Delta h}$$

to calculate the pressure at the top of Mt. Everest.