Calcium-45 has a half-life of 162.7 days. if four half-lives have elapsed, how much time has passed?

650.8 days

Explanation:

if the half-life is 162.7 days and the number of half-life days has elapsed 4 times.

To know the number of days that have passed we must multiply the half-life by 4

neutrons are in the middle of the atom and the electrons, which are much smaller.