In 4 years, 30% of a radioactive element decays. Find its half-life. (Round your answer to one decimal place.)

Question

_____ yr

Answer 1

(1/2)^(4/k) = 0.7
Now just solve for k.

Expect a number greater than 4, since if the half-life were 4, then 50% would decay in 4 years.

Answer 2

In other words
(1/2)^([Years]/X)=[% remaining]
Solve for X