Question

Strontium-85, used for bone scans, has a half-life of 64 {\rm d}.How long will it take for the radiation level of strontium-85 to drop to \frac{1}{8} of its original level?

Answer 1

I assume the half life is 64 days and you want to reduce the radiation to 1/8 of its original value.
k = 0.693/t_1/2 and substitute this into the below equation.
ln(No/N) = kt.
No = 100 (an arbitrary number I picked).
N = 12.5 (100/8) = 12.5
k from above.
Solve for t in days.