Question 856069
<pre>
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: {{{2*25!/26!}}} = {{{(2*25*24*23*""*""*3*2*1)/ (26*25*24*23*""*""*3*2*1)}}} = {{{(2*cross(25*24*23*""*""*3*2*1))/ (26*cross(25*24*23*""*""*3*2*1))}}} = {{{2/26}}} = {{{1/13}}}

Edwin</pre>