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.