I know the answer is 10, but the problem is I can't find out the formula :/ if I only show the answer I won't get any credit and it'll be marked wrong.
5 per group.
person x = 1st person meeting everyone
x - 1, x - 2, x - 3, x-4 (no x - 5 because he/she is 5th person).
second person meeting everyone = y
y - 1, y -2, y - 3,
(no y -4 or y - 5 because he/she already met with person x and cannot meet with himself)
Meeting count: 7
third person meeting everyone = z
z - 1, z - 2
z has already met with y, z has already met with x, z cannot meet him/herself.
Meeting count: 9
fourth person meeting everyone = k
k has already met with x,y, and z. k cannot meet with himself
everyone has met with each other