FranchiseLeadLetterID FranchiseLeadID LetterID    Created
--------------------- --------------- ----------- -----------------------
1                     24              6           2007-05-05 16:58:00
2                     25              6           2007-05-05 16:58:00
3                     25              5           2007-05-06 14:18:00
4                     25              4           2007-05-06 14:18:00


How do I get the group-wise maximum LetterID (based off 'Created') for the FranchiseLeadID ?