Unable to obtain a true addition with- odd excel fuction.

[Sedge]

Hi, I hope you can give me some help...I have a spreadsheet that contains a simple column of plus numbers and minus numbers.. In the event of me placing a number into cell a18, the sum of the column indicates an additional (point) .0000000001 into the equation. I have replicated the column on another computer with the same result. To negate a virus, I have done a Microsoft office 365 repair and also tried a fresh install with no satisfaction.

So when I do a balance in the spreadsheet of course I get an error. Interested in a solution. Thank you for any assistance. Cheers Sedge.