We are given : Per ticket cost = $7.
Processing fee of each order = $3.
Total cost of order including number of tickets costs = 3 + 7* number of tickets.
We are given table : Tickets | Price
1 | 10
2 | 17
4 | 31
5 | 38
10 | 73.
Let us rewrite that table according to the rule 3 + 7* number of tickets.
Tickets | Price
1 | 3 + 7*1 = 10
2 | 3 + 7*2 =17
4 | 3 + 7*4 = 31
5 | 3 + 7*5 = 38
10 | 3 + 7*10 = 73.
We can see that costs are proportional accoriding to the rule :
Total cost of an order = 3 + 7* number of tickets.
Let us explain it by one more way..
We can explain it by applying slope formula.

Plugging values in above formula by taking (1,10) and (2,17) coordinates form above table, we get

By taking (2,17) and (4,31) coordinates form above table, we get

We got same ratios for slope 7/1.
Therefore, we can say the cost of an order proportional to the number of tickets ordered.