The amount of time it takes for half of a sample to disappear is:

Here are the answers to your questions:

**Question 14: The amount of time it takes for half of a sample to disappear is:**
- **Answer:** d) half-life

**Question 15: The half-life for a 100-gram sample of radioactive element X is 5 days. How much of element X remains after 10 days have passed?**
- After 5 days, 50 grams will remain (half of 100 grams).
- After another 5 days (10 days total), half of 50 grams remains, which is 25 grams.
- **Answer:** c) 25 g

**Question 16: Sodium-24 has a half-life of approximately 15 hours. If only one-eighth of the sodium-24 remains, about how much time has passed?**
- One-eighth of the original amount means that three half-lives have passed, since:
- After 1 half-life: 1/2 remains
- After 2 half-lives: 1/4 remains
- After 3 half-lives: 1/8 remains
- Since each half-life is 15 hours, the total time is 3 * 15 hours = 45 hours.
- **Answer:** b) 45 hours