measures 200 counts per minute coming from a radioactive source
At 3 o'clock, she finds that this
has dropped to 25 counts per minute.
What is the half-life of the radioactive
The half-life is 1 hour.
After 3 hours, the count rate has fallen
from 200 to 25 counts per minute - in other words, it's fallen
to one eighth of where it started.
Count on your fingers:-
so one eighth of the count rate means that three half lives
If 3 half-lives are 3 hours, then the
half-life is one hour.