Question 874351
 Two taps A and B can fill a tank in 20 min and 30 min, respectively. An outlet pipe C can empty the full tank in 15 min. A, B and C are opened alternatively, each for 1 min. How long will the tank to be filled?
tap A can fill the tank in   2o minutes 
in 1 minute  Tap Ait can fill tank  1/20 parts 

Tap  B can fill the tank in 30 minutes
in 1 minutes tap B  can fill 1/30 parts 
Tap C can empty tank in  15 minutes 
Tap c can empty in 1 minutes  1/15 part 
Tap A ,B C together can fill  tank =1/20 +1/30-1/15 =1/60 parts 
therefore full tank can fill up if all three taps are on together = 60 minutes 
tank will get full if A B C are open alternatively 1 minutes = 60 X3 =180 minutes  Or 3 hours