Answer:
125 mg
Explanation:
Split into the day to have a better understanding
Day 0: 1000 (Nothing has happened yet)
Day 8: 500 (The sample has halved since it's reached it's half life once)
Day 16: 250 (Another 8 days passed so halved again)
Day 24: 125 (Another 8 days passed so halved again)
So the amount left is 125 mg