This answer is not correct. You probably thought that the mixture would be the same as 10 moles of a single gas. Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the pressures that each gas would exert if it were present alone. This means that you have to add the number of moles to get 20 moles. PV = (n_oxygen + n_nitrogen)RT