N molecules, each of mass m of gas A, and 2N molecules, each of mass 2m of gas  B, are contained in the same vessel which is maintained at a temperature T. The  mean square of the velocity of the molecules of B type is denoted by $v_B^2$  and the  mean square of the x-component of the velocity of A type is denoted by $v_A^2$ . The  ratio  $\frac{v_B^2}{v_A^2}=\frac{N}{3}$ . Find N.