The half-life of an element is 4.5e10 yr. How long does it take a sample of the element to decay to 3/5 of its original mass?
Use equation A=A(subzero)* (.5)^(t/T)
where A(subzero) is the initial amount. T is the half-life, and t is the time.
The solution shows how to calculate the time it takes to decay to 0.6 of its original mass for an exponential decay.