Hi!

It's a rounding issue caused by the SQRT function.

Try reversing the equation and you'll get the opposite result of -3.5E-15

=SQRT(24.6-3*8.2)

If you highlight this portion in the formula bar: 3*8.2-24.6, and then
press F9, you get a result of 0. But, as you've discovered, if you use the
formula auditing tools and evaluate the formula, 3*8.2-24.6, evaluates
to -3.5E-15.

So, try this:

=SQRT(ROUND(3*8.2,1)-24.6)

Maybe Jerry Lewis will see this post and explain it for us.

Biff

"big_ears" wrote in message
...
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.