We have `N_2` and `O_2` in the gas mixture. Mole fraction of `N_2` is 0.7
`X_(N_2) = 0.7`
Since mole fraction of both gasses add up to 1;
`X_(O_2) = 1-0.7 = 0.3`
Total pressure `(P_T) = 1.42atm`
Partial pressure = (mole fraction)`xx` (total pressure)
`P_(O_2) = X_(O_2)xxP_T`
`P_(O_2) = 0.3xx1.42 = 0.426atm`
So the partial pressure of `O_2` is 0.426atm.
Both gasses act as ideal gasses.