Excel stores it's numbers to a 15 Decimal Place precision, so it's
probably an internal rounding problem - if you force excel to round by
using the round function, you will resolve this

=round(ak66,2)=round(etc etc etc,2)

Hi

Your formula looks a bit odd! It is saying that if B66 is a number, then
$AK66=($P66-$V66+$AA66+$AB66-$AF66).
Eh?
Are you trying to compare AK66 to $P66-$V66+$AA66+$AB66-$AF66? See if this
does what you want:
=IF(AND(ISNUMBER($B66),$AK66=$P66-$V66+$AA66+$AB66-$AF66),"Yes","No")
This is untested - and might not be what you want anyway!

Andy.

I know of no documented instance of basic arithmetic errors in Excel.

Excel and almost all other computer software does binary arithmetic.  Most
terminating decimal fractions (including 650.38) are non-terminating binary
fractions that can only be approximated (just as 1/3 can only be approximated
as a decimal fraction).

Likely your calculation simply resulted in a different binary approximation
to 650.38 due to the binary approximations to the initial inputs.  With
addition/subtraction, you can simply round the final result to smallest
decimal place of any of the inputs without doing violence to the calculation.

My D2D function at
http://groups.google.com/group/microsoft.public.excel/msg/b106871cf92f8465
will allow you to more closely examine the approximations involved, if you
so desire.

Jerry