What is the half-life of isotope X? How many radioactive atoms of isotope X are left after
3 half-lives?
How many half-lives must have occurred if only 7
atoms of isotope X are left?

4

Explanation:

What is the half-life of isotope X?  25 seconds

How many radioactive atoms of isotope X are left after 3 half-lives? 13

How many half-lives must have occurred if only 7 atoms of isotope X are left? 4

Explanation:

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.