Question

A mad scientist has been apprehended in their secret laboratory in some remote woods. The place is severely contaminated with polonium-210 which has a half-life of 138.4 days. A geiger counter used on scene shows 140 decays per second. 2 decays per second would be considered safe / equivalent to background radiation. How long until the site is safe?

Answer 1

$$A(t)=A_0\cdot2^{-\frac{t}{t_{0.5}}}\\2=140\cdot2^{-\frac{t}{138.4}}\\t=848.3\ days$$