Question

A scientist begins with 100 mg of a radioactive substance. After 6 days, it has decayed to 60 mg. How long after the process began will it take to decay to 10 mg?

Answer 1

For radioactive decay:
N = N(0) x exp(-λt)
ln(6/10) = -6λ
λ = 0.085
For the amount to become 10mg:
10 = 100exp(-0.085t)
(ln(1/10)) / -0.085 = t
t = 27.1 days