ANSWER:
10:00 AM- 10:30 AM- 11:00 AM- 11:30 AM- 12:00 M- 12:30 PM- 1:00 PM- 1:30 PM- 2:00 PM- 2:30 PM- 3:00 PM- 3:30 PM- 4:00 PM- 4:30 PM- 5:00 PM
Explanation:
The least common multiple of 10 min (chemistry presentation) and 6 min (recycling presentation) min is 30 min. So, the two presentations have the same starting time every half hour.
Therefore, presentations start at the same time at:
10:00 AM
10:30 AM
11:00 AM
11:30 AM
12:00 M
12:30 PM
1:00 PM
1:30 PM
2:00 PM
2:30 PM
3:00 PM
3:30 PM
4:00 PM
4:30 PM
5:00 PM