ALONE
First we get the numerator of the probability:
We can choose the first letter 2 ways, either an A or an O.
That's 2 ways to choose the first letter.
For each of those 2 ways to choose the first letter, there are 4 ways
to choose the 2nd letter. That's 2×4 ways or 8 ways to choose the
first two letters.
For each of those 2×4 or 8 ways to choose the first two letters, there
are 3 ways to choose the 3rd letter. That's 2×4×3 ways or 24 ways to
choose the first three letters.
For each of those 2×4×3 or 24 ways to choose the first three letters, there
are 2 ways to choose the 4th letter. That's 2×4×3×2 ways or 48 ways to
choose the first four letters.
For each of those 2×4×3×2 or 48 ways to choose the first four letters, there
is just 1 way to choose the 5th letter. That's 2×4×3×2×1 ways or still
48 ways to choose all the .
Now we get the denominator. It's like the numerator except that there are 5
ways to choose the first letter. So the denominator is 5×4×3×2×1
So the desired probability is
2×4×3×2×1
--------- =
or .4
5×4×3×2×1
Edwin