9.) the isotope chromium-51 has a half-life of 28 days. if the original sample had a 51cr mass of 32.0 grams, what mass of 51cr would remain after 84 days

Explanation:

The half-life of chromium-51 (28 da) is the time it takes for half of it to decay.

After one half-life, half of the original amount will remain.  

After a second half-life, half of that amount will remain, and so on.  

We can construct a table as follows:  

We see that, after 84 da (three half-lives), ⅛ of the original mass remains.