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?
+37146 1/20 + 1/15 + 1/12 = 1/x solve for 'x' the time requird for all 3
