Sum of variable number of rows

[Aussie-Clive]

G'day All,

I reckon this should be easier than I'm finding it, but I just can't seem to find the right combination of functions to solve this.

In column A I will have an entry every 5 or 6 or 7 rows with blanks rows between.
In Column B starting at every row corresponding with an entry in Column A there will be a short column of figures could be 1 2 3 or four or more of them. (the're will always be a blank between the last number in a group in B before the next entry in A)
In column C starting at every Row corresponding to an entry in column A I want the sum of the group of figures in Column B.

But here's the rub.... The most important thing is that I can use the same formula all the way down column C. So click the one formula at the top and drag it all the way down 1000 rows.

I know this is easy for those with a bigger brain than mine.... but I'm stumped!