using the same algebra ...
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.