Greetings!
First of all, you need to find the sum of the total students:
24 + 26 + 28 = 78, so 78 students in total
If there has to be one adult for every 6 students, then you must divide the total students by 6.
78 ÷ 6 = 13.
So 13 teachers in total are needed. if there is already one teacher for each class then simply minus 3 teachers from the 13.
13 - 3 = 10
So 10 other adults are required!
Hope this helps!