To determine how much of the radioactive element X remains after 10 days, we can use the concept of half-lives.
The half-life of element X is 5 days, which means that every 5 days, half of the remaining amount of the substance decays.
After 5 days (one half-life):
- Initial amount: 100 g
- Remaining amount: \( 100 , \text{g} \div 2 = 50 , \text{g} \)
After another 5 days (a total of 10 days or two half-lives):
- Remaining amount: \( 50 , \text{g} \div 2 = 25 , \text{g} \)
Therefore, the amount of element X that remains after 10 days is 25 g.
The answer is c) 25 g.