Pipes A and B can fill a tank in 20 hours and 30 hours respectively and pipe C can empty the full tank in 40 hours. If all the pipes are opened together, how much time will he needed to make the tank full ?

$15\frac{1}{7} hours $ $13\frac{1}{7} hours $ $17\frac{1}{7} hours $ $19\frac{1}{7} hours $

$17\frac{1}{7} hours $