Cost of each ticket per person = $42
For groups of up to 8 people, the cost per ticket decreases by $3 for each person in the group.
Marco's ticket cost $30.
This means he paid [tex]42-30=12[/tex] dollars less.
Now when there are 8 members, the cost decreases by $3.
So, $12 will decrease, when there will be [tex]8\times4=32[/tex] members