Question 508959
There are {{{2}}} possible choices for the 1st letter
Repetitions are not allowed, so
there are {{{ 25 }}} choices for the 2nd letter
{{{ 24 }}} for the 3rd letter
{{{ 23 }}} for the 4th letter
Multiply possible choices to get all 
possible combinations of letters
{{{ 2*25*24*23 = 27600 }}}
There are 27600 possible 4-letter radio call signs