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I am trying to generate values in column D that appears every 3 rows. The first value in D are the sum of the first three values in A divided by the sum of the first three values in B. The second value in D should be in the fourth row that is the sum of rows 4-6 in A divided by sum of rows 4-6 in B and so on.

```
A B D
1 2 1+3+4/2+2+3
3 2
4 3
4 4 4+5+6/4+3+2
5 3
6 2
```

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