Chemical Equilibrium …...............................

Here according the equation, 1 mole of B will combine with 3 mole of A and so if x mole of B is consumed then it will combine with 3x mole of A and similarly we can calculate the number of mole of C and D formed(according to their stoichiometric coefficient: just think about this stoichiometry : its very basic and important!!!!)
Now from the question given that, at equilibrium,

(a/2)-3x = 2x

a/2= 5x,

x=a/10

so amount of A reacted=3x=3a/10
Initial amount of A=a/2

so % of A reacted= {3a/10}/{a/2} *100

=6/10 *100=60%