Question

After 3 days a sample of radon-222 has decayed to 58% of its original amount.(

Answer 1

Half-life = [elapsed time * log (2)] / [log (begng amt / endg amt)]
half-life =  (3 days * 0.30102999566) / log (1 / .58)
half-life =  0.903089987 / log  ( 1.724137931)
half-life =  0.903089987 / 0.23657200643
half-life =  3.8174 days

Source:
http://www.1728.org/halflife.htm