Surely you mean =$B$1*EXP(-(0.693*((A5-B2)/(30*365))))
which gives 197.460
What makes you think the "correct" value is 199.70?
If I replace 0.693 by LN(2) to improve the precision and 365 by 365.25, I
get only 197.464
I think you have been given the wrong answer! Take it from a retired chem.
prof that textbook errors are not rare.

Note that my formula does give a result that decrease as time does on (ie as
A5 increases)
--
Bernard V Liengme
www.stfx.ca/people/bliengme
remove caps from email

"me28" wrote in message
...
Thanks so much for a quick response. The information you provided is a
good
starting point but if possible let me provide additional information:

Initial Activity = 206 (cell B1)
Initial Date = 1/31/05 (cell B2)
Current Date = 12/1/06 (cell A5)
Half-Life = 30 (hard-coded in the formula)

My current formula is =$B$1*EXP(-(0.693*((B2-A5)/30)))
I need this to give me the decay value for 12/1/06. The correct value is
197.46, however my current formula returns 199.70 and is actually
increasing
each day when it should be decreasing.

Thanks for you time.

"Mike Middleton" wrote:

me28 -

With the value of A in cell A1, value of t in cell A2, and value of T
Half-Life in cell A3, in some other cell use

=A1*EXP(-0.693*A2/A3)

- Mike
http://www.mikemiddleton.com


"me28" wrote in message
...
I need to create a worksheet that takes an active value and calculates
the
timed decay. The formula is A e-(0.693t/T½) (e to the power of
(0.693t,
though LN may work). Value of A = Initial Value, T = Timed decay.