Awater tank can be filled by an inlet pipe in 6 6 hours. it takes 4 4 times as long for the outlet pipe to empty the tank. how long will it take to fill the tank if both pipes are open?

To fill:
6 hours  = 1 tank
1 hour = 1/6 tank

To drain:
It takes 4 times as long, therefore 4 x 6 = 24
24 hours = 1 tank
1hour = 1/24 tank

If both pipes are open 
1 hour = 1/6 - 1/24 = 1/8 tank
1/8 tank = 1 hour
8/8 tank = 1 x 8 hour
1full tank = 8 hours