27. A ship is in longitude 54°00'W on a true course of 270°. The ship's clocks are on the proper time zone. At what longitude should the clocks be changed to maintain the proper zone time?

The clocks should be changed at 6730′W, the western boundary of the 60W time zone. Time zones are 15 of longitude wide, with boundaries 730′ east and west of the standard meridian. Since the ship is traveling west from 54W, it will cross the zone boundary at 6730′W, requiring a one-hour clock adjustment.