casting: sand moulding and permeability number calculation

Question

 2000 cc. of air takes 1.5  minutes to pass through a standard specimen of sand (5.08 cm high and 5.08 cm in dia.). The manometer indicates pressure as  5g/cm². In this case the permeability number is

 

(a) 30               (b)42               (c)51                (d)67