Question

The half-life of chromium−51 is 27.8 days. How much of a 7.6−oz sample of chromium−51 will remain after 139 days?

Answer 1

Ending Amount = Beginning Amount / 2^n
where "n" is the number of half-lives
n = 139 / 27.8 = 5 half-lives
Ending Amount = 7.6 oz / 2^5
Ending Amount = 7.6 oz / 32
Ending Amount = .2375 ounces

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