beginning of content:
SAT Subject Test
The half-life of one isotope of radium is about 
years. In a given sample of this isotope, 
of the radium atoms will decay in a time most nearly equal to
Select an Answer
years
Correct Answer:
No
years
Correct Answer:
No
years
Correct Answer:
No
years
Correct Answer:
No
years
Correct Answer:
Yes
View Correct Answer
During each half-life, an average of 
of the nuclei present will decay. So after one half-life only about of the radium nuclei remain. After two half-lives the sample contains of , or about 
of the original number of radium nuclei. In general, the number of nuclei remaining after 
half-lives have elapsed is
.Having of the radium nuclei decay means having 
of them remain, which is 
. So the answer is four half-lives, which is 
years.
Question Difficulty:
easy