# If 12.5 grams of the original sample of cesium-137 remained after 90.6 years, what was the mass of the original sample?

##### 2 Answers

#### Explanation:

half-life of caesium:

remaining mass after

remaining mass after

1 half-life = original mass

3 half-lives = original mass

original mass

divide by

original mass =

#### Explanation:

I like to use this equation:

Where the mass of the substance that remains is represented as a function of t, the time which has elapsed in years, and:

And in our case (we are solving for the initial amount,

So

and