For A + B ⇌C + D, if initially the concentrations of A and B are equal but at equilibrium, concentration of D is twice that of A.  The equilibrium constant of the reaction is

A + B ⇌ C + D1          1            0         0    (Given conditions)1-x       1-x         x         x    (Equilibrium conditions)Given ,           D=2A x = 2(1-x) x=2-2x x=23A=1-23=13=B  :   C = D =x=23kC=CDAB=23×2313×13=4