Mary and David can do a job alone for 10 days and 15 days respectively. At first 4 days alone then David went to work alone for 5 days. Maria completed the work in 4 days. If Mary, David, Maria would finish the work together in three days.

The serenity and the mystic are sail boats. the serenity and the mystic start at the same point and travel away from each other in opposite directions. the serenity travels at 16 mph and the mystic travels at 19 mph. how far apart will they be in 3 hours?