Use division to find the per unit cost of each set. If 20 tickets cost $6, then each ticket costs 20/6 which is $3.33. 40 tickets costs $10, each ticket costs 40/10 which is $4. 40 tickets for $10 costs $4 (already done), and 50 tickets costs $15 and each one costs 50/15 which is 3.33. And course the third choice is a combination of the previous 2, so they are all 3 the same.