Well, the simple way to do it (it seems to me) would be in four steps:

1) In B1: =A1/2
2) In B2: =A2/2+B1
3) Copy the formula in B2 to B3:B5

Now you have the first block of five. You don't have to repeat all three steps throughout the rest of the worksheet; instead, just do this:

4) Copy the block B1:B5 to B6:B20 (or whatever).