How many grams of sodium-24 will remain at the end of 4 half-lives?
At the end of 4 half-lives, only 1/16th (1/2^4) of the original amount of sodium-24 will remain. If you start with 100 grams, at the end of 4 half-lives, you will have 6.25 grams remaining (100 grams * 1/2^4).