Marking data sets with non-blank cells
 

Hello,
I am working on a spreadsheet where the presence of text in the left-hand
column indicates a new set of data but am not quite sure how to work this out:

A | B | C | D
1 | x -
2 | -
3 | - - -
4 | x -
5 | -
6 | -
7 | - - -

- = numbers, x = text string (any text)

D3=IF(C30,SUM(B1:B3),0). Columns A and C are like the markers for the start
and end of a data set; the first row in column A containing text marks the
start and the first instance of a number in column C marks the end of this
data set. Cells in column D will display the sum of column B in this data set.

The problem is that the size of the data set is not always constant, so the
first set above contains 3 rows (A1:D3), while the second contains 4. This is
the part where I am stuck at. I have heard of using VLOOKUP and MATCH but
neither of them are really working at the moment.

Thanks in advance for any help!