Hi, Rick,

I am trying to figure how many iterations it takes 100 to go below 50 with a
decay rate of 20%. i.e. 100-(100*.8)=80 then 80-(80*.8)=64...and so on. I
am trying to set this up in one cell. Thanks.

I think you're looking for this:

=LN(0.5)/LN(1-A1)

Where your rate, 0.2 in this case, is stored in A1.

Peace,
--Carl