Section+4.5

 **2. Trains** You can use number rods to build “trains” that all share a common length. A “train of length 5” is a row of rods that have a combined length of 5.

====For this question there are different ways to approach this problem. In the end not much was done with this problem because we lost time and the bell rang. However one of the ideas I came up with is that for each set starting with 5 we can systematically count how many there are for each type of train tracks. Clearly with track 5 there is only one possibility. With 4 there is two, it can either be 4-1, or 1-4. So this is just one of the ways that the problem can be solved. It is clearly a very difficult problem that will be tackled later on in the unit.====