The capacity of the tank is 60 units(LCM 12,30)
=>Efficiency of inlet pipe = 60/12 = 5hrs=>It can fill 5 units of the empty tank working alone in 1hr.
=>Efficiency of outlet pipe = 60/30 = - 2 (negative sign because its an outlet pipe)
=>It can drain 2 units of the filled tank in 1 hrs alone
=>Working together, their efficiency is 5-2 =3 units/hr
=>Working together, they can fill 3 units of the empty tank in 1 hr
=>Working together, they can fill 60 units in 60/3 = 20 hrs (option c)