Problem 1. On a certain day in a harbor, low tide will occur at 12 noon and the depth of the water will be 3-m. The subsequent high tide will occur at 6:20pm and the water will then be 15-m deep. A ship which needs 6-m depth of water wishes to use the harbor. Find to the nearest minute the earliest time it can enter the harbor on this day and the time by which it must leave.