Chemistry Daily Challenge 17-July-2015

Chemistry Level 3

One mole of $\ce{N2O4}(g)$ at 300 K is kept in a closed container at 1 atm. It is heated to 600 K when 20% by mass of $\ce{N2O4}$ decomposed to $\ce{NO2}(g).$ What is the resultant pressure in atm?