Rewrite and order 13/15 and 5/6
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There are 6 other positions in the string, each with 26 choices. So if you fix BO as the first two letters, there are 
possible strings that you can make.
If BO is at the end of the string, you still have possible strings.
Together, then, you have
possible strings.
If BO is at the end of the string, you still have
Together, then, you have