7. 4 7 decays with a half-life of about 53 days. It is produced in the upper atmosphere, and filters down onto the Earth's surface. If a plant leaf is detected to have 350 decays/s of 4 7. Calculate the time need to wait for the decay rate to drop to 15 per second.

:Be decays with a halflife of about 53 days. It is produced in the upper atmosphere, and lters down onto the Earth's surface. If a plant leaf is detected to have 350 decays/s of 136. Calculate the time need to wait for the decay rate to drop to 15 per second