Answer:
They will need 13 buses
If you have 18 in three of the classes you will need three buses per class
Each van holds 5 kids divide 18 by 5 you get 3 (plus a remainder we will get back to that later)
and 2 vans for the class of 10
So far we are at 11 buses
But there is that remainder you will need 2 more for them
So that puts us at 13 buses
Hope this helps