The half life of Thorium 227 is 18.72 days. It decays by alpha emission to Radium 223, an alpha emitter whose half life is 11.43 days. A particular sample contains 10^6 atoms of Thorium 227 and no Radium 223 at time t = 0.
(a) How many atoms of each type will be in the sample at t = 15 days?
(b) At what time will the number of atoms of each type be equal?