A boat travelling with and against a current

A boat travels \(36\text{ km}\) downstream in \(2\) hours and returns the same \(36\text{ km}\) upstream in \(3\) hours. Assume the boat's speed in still water, \(b\text{ km/h}\), and the speed of the current, \(c\text{ km/h}\), are constant.

1. Explain why the downstream speed is \((b+c)\text{ km/h}\), and form two equations in \(b\) and \(c\).[2]
2. Find \(b\) and \(c\).[2]
3. On another day the current is twice as fast. Find the time for the upstream journey, and state one assumption you have made.[2]
4. Explain why the model fails if the current is \(15\text{ km/h}\) or faster.[1]