First, divide 638 but 6 the best you can having remaining numbers
106 1.) divide
6 | 638
- 636 2.) subtract
2
There are 2 remaining students ( and yes every student must have a group)
So you have to create a another group for these 2 students making there be
107 tour groups
I did this to my best ability so i am so sorry if it is wrong but i hope this helps