Every 3 feet is $18. 18 divided by 3 equals 6.
24 divided by 6 equals 4. (So the second one is correct)
30 divided by 6 is 5, so the answer is not 30. it'd be 36.
48 divided by 6 is 8, so it is not 7.
72 divided by 6 is 12. But 6 multiplied by 9 equals 52. So, your answer is 52.
Hoped this helped,
-Anime