In this question, we have to calculate the final mass of a radioactive substance with a half-life of 6 hours, 24 hours later, and we can do that by calculating decay by decay until we reach 24 hours:
100 grams = 0 hours
50 grams = 6 hours
25 grams = 12 hours
12.5 grams = 18 hours
6.25 grams = 24 hours, therefore after 24 hours, we will still have 6.25 grams of this radioactive element