What fraction would be left after 7 half lives?

Two friends at different locations want to communicate with each other by sending low energy signals. which of the following methods can they use to communicate? a) produce x-rays using colliding electrons and send them to radios, which capture sound b) send messages using infrared radiation, which travel in the form of waves c) send radio waves through intervening media like radio and television d) produce sound waves using microwaves from heated objects

An empty fuel tank can still contain and therefore can be even more dangerous than one full of liquid fuel.

Which description best characterizes the motion of particles in a solid?