Question

Question #67ffa

Answer 1

`139.32` seconds

Explanation:

If only 4% of the original sample remains, we know that:

`100/2^n = 4`, where `n` is the number of half-lives elapsed.

Rearranging this gives

`2^n = 25`
`n = log_2(25)`
`n = 4.64386` half lives

Since a half-life of Radium-221 is 30 seconds, we know it must have taken 4.6439*30 seconds for 96% to decay, which is `139.32` seconds.