Question

If 40 milligrams of strontium-90 radioactively decays to 12 milligrams in 30 years, find its half-life (the number of years it takes until half of it remains). Use the formula A = p ⋅ e−kt, where p is the amount and A the (smaller) final amount.

Answer 1

12 = 40 e^(-kt)
.3 = e^-30 k
ln .3 = -30 k
-1.20 = -30 k
k = .04013
so
.5 = e^-.04013 t
-.6931 = -.04013 t
t = 17.3 years