Re: HSC 2013 4U Marathon
Consider 3-letter string starting with A, B, C, D. There are 4*26^2=2704. There are 26 for each EA, EB, EC, ED, EF, so 2704+26*5=2834. Then there's EGA,EGB,...,EGG, 7 of these. So 2841 in total.
(edit: assuming no counting mistake)