Painting Michael's fence
Three workers, Frank, Grace, and Hazel, have been hired to paint a fence. Frank alone can paint the fence in 4 hours, Grace in 3 hours, and Hazel in 2 hours. The workers alternate painting in the following manner: Frank works for 20 minutes, then Grace works for 20 minutes, and then Hazel works for 20 minutes, and the process repeats. How many minutes will it take the workers to finish painting the fence?
This problem is posed by Michael T.
Details and assumptions
Assume each of the workers works at a constant rate.