(a) In a sample of 20,000 atoms, if 400 decay in 8 seconds, what is the radioactivity, measured in mCi, of the sample?

(b) In order to produce a level of radioactivity of 1mCi, how many nuclei of ^99m{Tc} (decay constant λ = 3.22 x 10^-5 s^-1) must be present?

(c) A radioactive sample of ^99m{Tc} contains 10mCi activity at 9 am. What is the radioactivity of the sample at 12 pm on the same day?