If the passing of five half-lives leaves 25 mg of a strontium-90 sample, how much was present in the beginning?

800 milligrams strontium-90 sample was present in the beginning.

Explanation:

Formula used :

where,

Amount of sample left after n-half lives  = [A]

Initial amount of the sample =

n = number of half lives

We have:

Final amount of strontium-90 left after 5 half lives=[A]=25 mg,

Initial amount of the strontium-90 sample =

n = 5

800 milligrams strontium-90 sample was present in the beginning.