Answer:
5 buses
Explanation:
Mrs. Smith will need to reserve buses for 7 classes of Fourth graders, with 20 students in each class, for a total of
7 * 20 = <<7*20=140>>140 students.
Since 30 students fit on a bus, Mrs. Smith will need to reserve
140 / 30 = <<140/30=4.67>>4.67 buses.
Since Mrs. Smith can't reserve a partial bus, she will need to reserve 5 buses for the trip.