Answer: the group with 6 people.
Explanation:
If you divide the first group with 5 people. Divide 21/5= 4.2 per person.
If you divide the second group with 6 people. Divide 25/6= 4.16 repeating.
So the group with 6 people will have a lower cost per person.
Hope this helps ☝️☝☝