The half-life of a radioactive element is the time it takes for half of the substance to decay. In this case, the half-life of element X is 5 days.
After 5 days (1 half-life), 50 grams of element X will remain from the original 100 grams (half of 100 g).
After another 5 days (a total of 10 days or 2 half-lives), the remaining 50 grams will again be halved. Therefore:
50 g (after 1 half-life) → 25 g (after 2 half-lives)
So, after 10 days, 25 grams of element X will remain.
The correct answer is b) 25 g.