The sample started with 45 billion atoms of tritium, after 4 years the sample had 36 billion atoms of Tritium , calculate the percentage of the tritium in the sample that remains after 4 years

Consider a system to be two train cars traveling toward each other. what is the total momentum of the system before the train cars collide?   kg • what must the total momentum of the system be after the train cars collide?   kg •