For the seventh-grade field trip with 225 students, 15 adults are needed. This is determined by the requirement of one adult per 15 students. The calculation ensures a safe and organized trip, with each adult responsible for supervising a group of 15 students.
To determine the number of adults needed for the seventh-grade trip, we'll use the information that one adult is required for every 15 students. First, find the number of trips each grade level requires by dividing the number of students by 15:
1. **Fifth Grade:**

2. **Sixth Grade:**

3. **Seventh Grade:**

Now, the total number of adults needed for the seventh-grade trip is the same as the number of trips, which is 15. Therefore, 15 adults are required to accompany the seventh-grade students on their field trip. The calculation ensures that there is at least one adult for every 15 students, meeting the specified ratio for a safe and organized trip.