You can use rods of integer sizes to build "trains"
that all share a common length. A "train of length 5"
is a row of rods whose combined length is 5.
Notice that the 1-2-2 train and the 2-1-2 train contain the same rods
but are listed separately. If you use identical rods in a different order,
this is a separate train.
You can find additional hints, results, and other
resources for this warm up problem in
Trains makes a good warm up for the Simplex Lock project.