what is LOOKUP(9.99999999999999E+307

Without getting too complex...

9.99999999999999E+307

is generally regarded as the biggest number you could have in any XL spreadsheet and is often used to find the last number in a given range using LOOKUP (in the same way LOOKUP(REPT("z",255)...) is used to find the last text value in a given range.

In Aladin's formula below:

=LOOKUP(9.99999999999999E+307,INDEX($C$2:$IV$25,MATCH(A28,$A$2:$A$25,0),0))

It is used to return the last number within the INDEX range.

The INDEX range is itself dynamic and set to be C:IV for the row in which A28 occurs in A2:A25 -- so if looking at row 8 the above will find the last numerical entry in C8:IV8 and return it.

For a better explanation -)) see Aladin's own explanation here:
http://www.mrexcel.com/forum/showthread.php?t=102091