The tides in a harbour follow a 12 hour cycle. A captain wants to take his ship out to catch some fish but needs 5 metres of water to clear a sandbar at the entrance to the harbour. At low tide, the depth of the water over the bar is 2 metres and, at high tide, the depth of the water is 9 metres. Assume that it is currently low tide.

a) When is the first time that the captain can leave the harbour?

b) How long does the captain have before the water over the bar is too shallow to return? c) How quickly is the water rising/falling at each of these two times?