River Crossing

The introductory problem is the classic river crossing problem.  

Encourage students to consider the different options for the first boat trip.
There is only one possible safe choice for the initial crossing!

After that there is only one trip in which there is a choice.

The second problem can be taken further as it invites students to generalise to a larger, unknown number of adults.  Students should be encouraged to work systematically so that the patterns in their results are easier to spot.

There is a simpler case (of one adult and two children) which is not mentioned in the problem.