Are you sure this is blocks of 20? The range A21:A41 has 21 cells, as does A42:A62
I will assume all blocks are 20 cells and you want the data compiled in a column starting in the first row.

Put a formula in cell H1 with this value
=SUM(OFFSET($A$1,(ROW()-1)*20,0,20,1))

Then copy this formula down for as many rows as you need.

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Andrew Lockton
Chrysalis Design, Melbourne Australia