Try this ARRAY formula

=IF(C1="","",SUM(A1:INDEX(A:A,MAX(1,MIN(IF($C$1:$C 1="",ROW($B$1:$B1)))))))

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HTH

Bob

"Malika" wrote in message
...
Hello,
I am using Excel 2007. Column A records individual sales. Column C
records
the date that bulk payments are made to suppliers. When a date is entered
into Column C, I would like Column B to sum the value in Column A up to
the
previous bulk payment. For example, when the date is entered into C7, B7
should sum A7:A2.

A B C
1 14.40 14.40 15/03/2010
2 9.00
3 9.00
4 9.00
5 9.00
6 9.00
7 9.00 54.00 25/03/2010

Any assistance would be appreciated.