Hello,
The answer to #18 will be 4 since for every x-value, the y-value increases by 0.5. It's just that they skip some of the x-values tricking you.
So, let's use the first column set of answers to find the price for 4 tickets WITHOUT the service fee. If the service fee is $8.50, we need to subtract that from the total amount $56.50 to find out the prices for the tickets without the service fee: $48.00
Divide that by 4 so you can find out the number for just one ticket and you'll find that it costs $12.00 for a ticket without the service fee.
Now finding how much a single ticket costs, let's plug it in:
9 x $12.00 = $108.00 AND now you have to add the $8.50 as the service fee:
$108.00 + $8.50 = $116.50 dollars
Hope this helps!
May