Given the nets of 4 cubes with the faces coloured in 4 colours, build a tower so that on each vertical wall no colour is repeated, that is all 4 colours appear.
Label the joints and legs of these graph theory caterpillars so that the vertex sums are all equal.
Label this plum tree graph to make it totally magic!
The use of set terminology here should not hold anyone up as they can
find the definition in the Thesaurus. This is an exercise in
combinatorics and it also gives a method for solving the problem about
the sequence of subsets.