rate and optical rotation

its 52.5 ml
the optical rotation gets halved in 30 min and optical rotation is directly proportional to concentration .therefore we can say thatconcentration becomes halved in 30 min so 30 min is the half time of the rxn

let assume the concentration of A at starting point is x therefore after 30 min it is x/2 the concentration of B at 30 min is x(double of A) and so we can calculate the concentration of B at 90 min is 7x/4 so the ans is 52.5 ml