Answer:
They need 8 vans.
Explanation:
8 × 9 is 72. If you have 8 vans that carry 9 students each, you will have room for 72 students. Then you have enough room for 70 students.
If you only have 7 vans you can only transport 7 × 9, or 63 students. That is not enough. They need 8 vans.