since you always round up when you estimate in the middle lets say theres 420 tickets. Each is 7.95, but we'll round it to 8.00. 420x8.=3,360. After that part we need to find about how much tickets were refunded. So we round 18 to 20 and each ticket is still 8$. 20x8=160. Finally in all, we do 3,360-160=3200. Its an overestimate because we rounded up. Im not sure what else youre asking but thats the basic answer.
