Question

What is the half-life of radon-222 if a sample initially contains 120.0 mg and only 18.5 mg after 11.2 days?

Answer 1

First, calculate the decay constant k.

\[A=A _{o}e ^{kt}\]
Where A is the final amount, A -naught the initial amount, and t time.

or check this out. It gives formulas as well as a calculator to check your work.

Answer 2

Ooops. This is what you'll want to check out:
http://www.1728.org/halflife.htm