If a radiographic substance's half-life is 20 years, how much of a 40 gram sample will be left after 60 years?

A. 5 grams

B. 10 grams

C. 20 grams

D. 30 grams

A because 60 divided by 20 is 3 so it is going through 3 stages of half life. 40/2=20 which is one half life then 20/2=10 which is the second half life then 10/2=5 which is the 3rd and final half life