Weird anomaly I found today, any ideas?

[Rusty]

After building this wizz bang data importer/manipulator/exporter (Won't bore you with the details) for a client, and it all running smoothly, it bugged out today...

Got bounced back to me as an issue, and after a while of analysing the source, process and output, I came across this weird anomaly and thought I would get your opinons (if you please)

1) Open a new workbook.
2) Enter into any cells the following: eg:

A1: -9.88
A2: 9.60
A3: = A1 + A2

Logical answer is -0.28, no?

Now change the formatting of the cells to be a number with 15 decimal places.

The result in cell A3 is now: -0.280000000000001

WTF?

Where did that 1 come from? Can anyone explain that to me?

 

[Jon von der Heyden]

I think Chip covers this topic here: http://www.cpearson.com/excel/rounding.htm

 

[Rusty]

Cool, kind of makes sense.

Thanks for the link. Much appreciated.