Say I have this data in my Excel speadsheet which lists my 40
best-selling products.

Sales rank      Number sold by that group
1-10              30
11-20             40
21-30             30
31-40             35

But say I want to estimate what were the total sales of items 1
through 25. It would be an estimation, because I don't have the data
broken down that way. How would I do that in Excel?

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Clarification of Question by boboleo-ga on 09 Aug 2006 20:12 PDT

(The real data isn't quite this tidy, of course.)