to shovel all of the snow on his driveway, Kevin needs 12 hours. Individually, Dave needs 8 hours to shovel all of Kevin's snow, John needs 6 hours to shovel all of Kevin's snow, and Alison needs 4 hours to shovel all of Kevin's snow. If Kevin, Dave, John and Allison all work together, how many minutes do they need to shovel all of Kevin's snow.