Interpretation: John takes 40 min, Asscab 30 min and Jack 24 min to type a 20-page document each, working alone. How long does it take if they all work together?
John completes a page in 2 mins, Asscab in 1.5 mins, Jack 1.2 mins. Three pages would be completed in 4.7 minutes, so 20 pages would be completed in 4.7*20/3=94/3=31 1/3 minutes (31 mins 20 seconds).