Question

a certain radioactive substance loses one third of its original mass in 5 days. Find its half-life

Answer 1

half-life = elapsed time * log(2)/[log(beginning amt/ending amt)]
half-life = 5 days * .30103 / log (1/.3333)
half-life = 1.51 / log(3)
half-life = 1.51 / 0.48
Half-life = 3.15 days

A good page to read about half-life:
http://1728.org/halflife.htm