The half life strontium-90 is 28.8 years. How much of a 325 gram sample is remaining after 4 of half lives? How old is the sample?
After four half lives 20.3125 is remaining and the sample is 115.2 years old.
We start with 325 and divide by two this accounts for one out the four half lives. Which is 162.5 once again we divide by two and we get 81.25 this is two out of the four half lives. The third time we divide we get 40.625 and we do this one last time and arrive to the answer of 20.3125. So now only 20.3125 from the original 325 gram sample remains. By now we have completed four cycles, to get the total amount of years we simply multiply 28.8 by 4 and we get 115.2.