7/4Be decays with a half-life of about 53d. It is produced in the upper atmosphere, and filters down onto the Earth’s surface. If a plant leaf is detected to have 450 decays/s of 7/4Be,
(a) How long do we have to wait for the decay rate to drop to 15 per second?
(b) Estimate the initial mass of 7/4Be on the leaf.