​please answer this in detail

Q. A sample of mixture of A(g) , B(g) and C(g) under equilibrium has mean molecular mass equal to 80. The equilibrium is :

                   A(g) $\underset{}{\rightleftharpoons }$B(g) + C(g)
     If initially 4 mole of  'A' gas is present then total number of mole at equilibrium is:
                                [M_A  100, M_B = 60 M_C  40]

     (1) 5                     (2) 2                         (3) 6                     (4) 4