Question

If the half life for strontium-90 is approximately 28 years, then what is the whole life for strontium-90?

Answer 1

Set this up as

10 = 40 (1/2)t/28

and solve for t in years.

10/40 = (1/2)t/28

log(0.25) = (t/28) log(0.5)

t = 28 log(0.25) / log(0.5) years = 56 years

Test the answer:

56 years = 2(28 years) = 2 half-lives, so the remaining amount should be 1/2 of 1/2 of the initial amount, i.e. 1/4 of the initial amount, and 10 mg = 1/4 of 40 mg.