The number of ways to arrange the 25 things where "AB" is one thing, and the other letters are the other 24 things, is: {AB,C,D,E,F,...,X,Y,Z} is 25! The number of ways to arrange the 25 things where "BA" is one thing, and the other letters are the other 24 things, is: {BA,C,D,E,F,...,X,Y,Z} is also 25! So that 2×25! out of the number of ways to arrange the 26 things: {A,B,C,D,E,F,...,X,Y,Z} which is 26! Answer:= = = = Edwin