physics
posted by ronny .
if 5/6th of a radioactive sample decay in 70 years and 1/6th remain. how can you find the half life without knowing the decay constant can you help?

The fraction (1/6) that remains after 70 years satisfies the relation:
1/6 = 2^(t/T),
where T is the half life and t = 70 years.
Take logs of both sides to solve for T. I'll use logs to base e but it doesn't matter
1.7918 = 70/T ln 2 = 48.52/T
T = 27.1 years