Physics
posted by Beth .
A first order exponential decay can be written as
A(t)= Ae^(t/r)
where A(t) is the amount (of substance) after time t, A is the initial amount at time t=0 and r is the decay time.
The fall time is defined as the time in which A(t) falls from 90% to 10% of its initial value. Find the relationship between the fall time of an exponential decay and r.

I will start it off
.9=e^t1/r
ln .9 = t1/r
likewise..
ln .1=t2/r
tfall=t2t1, well you do that...