I am needing some help on this situation. I have the data structured
in this way:

A B C
Res ID Cost Activity Date
1 1234 $20 1/1/2010
2 1234 $25 1/20/2010
3 7432 $15 2/2/2010
4 2574 $45 10/1/2009
5 7432 $65 1/2/2010

It shows by resource (Col A), Hourly Cost (Col B), and Activity date
(Col C).

Then I have a second sheet, with this format:

A B C
Res ID Cost Effective Date
1 1234 $15 12/30/2009
2 1234 $18 1/18/2010
3 7432 $12 1/31/2010
4 2574 $43 9/29/2009
5 7432 $67 12/31/2009

Same format, but it shows the standard cost. Effective Date represents
the day when this cost becomes available.

What I am trying to do is in the first sheet, add a column with the
current Cost (Sheet 2) at the moment of the transaction. Example:
Resource 1234, shows $20 cost on an activity on 1/1/2010, but the real
cost coming from sheet 2 is $15 (because it is since 30/12/2009 to
1/18/2010, when a new cost becomes effective).
Sheet 2 can contain two, three, four... x numbers of times the same
resource (with different effective date, of course).

Does somebody know how to calculate this??? In the meantime I am
trying with Index, Match, Sumproduct, Lookup.......

Thanks!
Cecilia