posted by Gajender Goswami .
An isotope has a 12.4 hour half life. If it has an activity of 30mCi at noon today, what was its activity at 3 p.m. yesterday?
the time involved is 9+12 = 21 hours, or 1.69 half-lives
so, we need to find P where
30 = P*(1/2)^1.69
P = 96.8