I like the 'question mark' at the end:
26 letters (English), 10 digits (0,1,...9)
26*26*10*10*10 = 676000
Now, this was for LLDDD (Letter Letter Digit Digit Digit)
It seems this is the order they want. Other wise, one should multiply by all the combinations in the order: LDLDD, DLLDD, etc ... (10 combinations), but again it seems they want this LLDDD
So 676000
