If the mass of a radioactive substance is 8 grams and it has a half life of 4 hours, how much mass remains after 8 hours?
A half life is defined as the period of time it takes for half of a radioactive substance to decay. Thus, if a substance has a half life of 4 hours, it will take 4 hours for half of the substance to decay.
Using this, we know there are 2 half lives of the substance in 8 hours and we have to divide the original mass of the substance in half twice:
There is a more versatile equation to solve problems like these, but since this problem gave us nice whole numbers to work with this method is just fine.