All of the stocks on the over-the-counter market are

[mastevano]

Here's another one that stumped me on the GMATPrerp practice test. I wish they provided answers!!

[Guest]

Max number of stock codes = No. of possible 4 letter codes + No. of possible 5 letter codes.

Now if the 4 letter code is abcd, a can be one of the 26 letters, b can be one of 26, similarly for c and d.
So, total = 26*26*26*26 = 26^4
No. of 5 letter codes = 26^5

Total = 26^4 + 26^5 = 26^4(1+26) = 27(26^4)

-givemeanid

[StaceyKoprince]

Perfect explanation, givemeanid!