Answer:
42
Explanation:
The Ferris wheel opens for 30 minutes at 11 a.m. Now, the first seat is filled at 11 a.m. Every minute a seat passes by the platform so every minute a seat is filled. This means that from 11:00 a.m. to 11:09 a.m. the seats are filled. At 11:10 a.m., the ride starts. From 11:10 to 11:30, the time duration is 21 minutes. Each minute a seat passes by the exit platform and 2 people leave the ride through the exit platform. Hence, a total of 42 people have ridden during this time.