Yes, I was playing around with that one... Exponential decay for activity... I'm not sure what else I can use... At one point, I also used n = n0 *exp (-0.693t/t12), but wasn't sure if the n given in the problem was actually number of atoms or just some arbitrary variable.
Fission product activity measured at the time t0 following the burst of a nuclear weapon is found to be a0. Show that the activity at the time t=7^nt0 is given approximately by a=a0/10^n. This is known as the 7-10 rule in civil defense.