Consider the points P =(0,0),Q = (1,0), R= (2,0), S =(3,0) in the xy-plane. Let A,B,C,D be four finite sets of colours(not necessarily distinct nor disjoint). In how many ways can P,Q,R be coloured bu colours in A,B,C respectively if adjacent points have to get different colours? In how many ways can P,Q,R,S be coloured by colours in A,B,C,D respectively if adjacent points have to get different colors? combinatorics unsolvedcombinatorics