I am attempting to find a value using a series of repeditive formulas. I am
getting an #NUM error, which I know is wrong.

I am attempting to calculate:
----------------------
A B
8.2 =SQRT(3*A-24.6)
----------------------

Now, I know that 3*8.2=24.6. I know that 24.6-24.6=0. I know that the
SQRT(0)=0, and Excel knows this as well.

I have looked back at the step-by-step troubleshooting in Excel, and for
some reason it takes (3*8.2-24.6) to be approximatly -3e^-15 or somthing in
that range. It thinks that it is a negative number, and won't find the SQRT
of it, but when I do it in step by steps it does it fine. It just can't
manage it in one go.

Why can't I do this and how do I fix it?

Thanks, I hope this is in the right place.