Answer:
4
Explanation:
53 students, 7 adults
Let S = 1 student and A = 1 adult
A van holds 6 people and must contain 1 adult, so a van must have 1 adult and up to 5 students.
Van 1:
A, S, S, S, S, S Total so far: 1 adult, 5 students
Van 2:
A, S, S, S, S, S Total so far: 2 adults, 10 students
Van 3:
A, S, S, S, S, S Total so far: 3 adults, 15 students
Van 4:
A, S, S, S, S, S Total so far: 4 adults, 20 students
Van 5:
A, S, S, S, S, S Total so far: 5 adults, 25 students
Van 6:
A, S, S, S, S, S Total so far: 6 adults, 30 students
Van 7:
A, S, S, S, S, S Total so far: 7 adults, 35 students
Now we ran out of adults.
53 - 35 = 18
There are still 18 students to be placed in vans.
Van 8:
A, S, S, S, S, S Total so far: 8 adults, 40 students; 1 extra adult
Van 9:
A, S, S, S, S, S Total so far: 9 adults, 45 students; 2 extra adults
Van 10:
A, S, S, S, S, S Total so far: 10 adults, 50 students; 3 extra adults
Van 11:
A, S, S, S Total so far: 11 adults, 53 students; 4 extra adults
4 more adults are needed