I have 3 columns, A, B & C with numbers in each and data (numbers only)
periodically gets added to the bottom row of each column. I am trying to set
up a formula in column B so that when the number 1 appears in column C, it
will sum the numbers in column A, starting from 1 row below the row in column
C where the number 1 last appears above and ending in the row that the
formula in Column B is located. The number 1 may appear in column C multiple
times and the cells in column C will only populate with either a 0 or 1. I
would like this process to be automated so that as new numbers are added at
the bottom it will automatically make these calculations. Does anyone know
how to make this work?

Here are some examples of what I am trying to do:


A B C
1 2 0
2 2 0
3 4 1
4 2 2 0
5 1 3 0
6 2 1
7 4 4 0
8 6 10 0
9 8 18 0
10 2 20 0


Example 1: The formula in B4 will sum A4:A4. (= 2)
Example 2: The formula in B5 will sum A4:A5. (= 3)
Example 3: The formula in B7 will sum A7:A7. (= 4)
Example 4: The formula in B8 will sum A7:A8. (= 10)
Example 5: The formula in B10 will sum A7:A10. (=20)

Can anyone help on this?

Thanks in advance