Cumbersome, but it works
=SUM(INDEX($B$1:$B$100,MATCH(C1,A:A,0)):INDEX($B$1:$B$100,
IF(MIN(IF(INDEX($A$1:$A$100,MATCH(C1,A:A,0)+1):A100<>"",ROW(INDIRECT(MATCH(C1,A:A,0)+1&":100"))))=0,100,MIN(IF(INDEX($A$1:$A$100,MATCH(C1,A:A,0)+1):A100<>"",ROW(INDIRECT(MATCH(C1,A:A,0)+1&":100"))))-1)))
as an array formula, commit with Ctrl-Shift-Enter, not just Enter

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HTH
Bob
(change the xxxx to gmail if mailing direct)
>I have a spreadsheet set up as the following:
>
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>
> Thanks!