Thermodynamics: Kays Rule

chaosturtle

Hello sir, would like some advice on Kays Rule in thermodynamis... it says a volume of .4m^3 of O2 at 300k and 8mpa , mix with .6m^3 of N2 at the same temp and pressure form a mixture at 300k and 8mpa.Determine the volume of the mixture using kays rule..

dont know how to start , i know the formulas for when the problem gives you kmol of the gas but im stuck here..