A sample of a material has 200 radioactive particles in it today. Your grandfather measured 400 radioactive particles in it 60 years ago. How many radioactive particles will the sample have 60 years from today?

Amount of radioactive particles left after 60 years = 100 particles.

Explanation:

Amount of radioactive particles before 60 years = 400

Amount of radioactive particles present today = 200

That is radio active particles reduced to half. That is 60 years is half life of this radio active material.

After 60 years this 200 radio active particles will reduce to half.

Amount of radioactive particles left after 60 years = 0.5 x 200 = 100 particles.