The graph below shows the radioactive decay of rubidium-87.

Question

What percentage of rubidium-87 atoms will be left after three half-lives?
A.
50 percent
B.
25 percent
C.
12.5 percent
D.
6.25 percent

Answer 1

C. 12.5 percent.

After one half-life, 50% of the rubidium-87 atoms will decay and 50% will remain. After two half-lives, another 50% of the remaining atoms will decay, leaving 25% of the original amount. After three half-lives, another 50% of the remaining atoms will decay, leaving 12.5% of the original amount.