You have 40 g of a radioisotope. If the half-life of this radioisotope is 4 days, how many grams will remain after 12 days?

the number of grams of radioisotope that have remained after 12 days if the half life if 4 days is calculated as below

calculate the number of half lives involved

12/4 = 3 half live are involved

40 after one half life = 40/2 = 20 g

20g after the second half life = 20/2 = 10g

10g after the third half life = 10/2 = 5 g

therefore 5 g will be remaining