# What percentage of strontium-90 remains in a sample after two first-order half-lives have passed?

May 31, 2018

25% or 1/4

#### Explanation:

A half- life is the time required for half of the radioactive substance to undergo radioactive decay. In two half lives first half of the original sample decays then in the second half life half of what is left decays.

Mathematically this is

$\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

 1/4 = 25%