To fill an empty cistern, we can use pipe A which would take 20 minutes, or we can use pipe B which would take 15 minutes, or we can use pipe C which would take 12 minutes.
If we turned on all three pipes, how long will it take to fill up the empty cistern?
+36915 1/20 + 1/15 + 1/12 = 1/x solve for x the amount of time required if all three are used
3/60 + 4/60 + 5/60 = 1/x
12/60 = 1/x
x = 60/12 = 5 minutes
