"18" is just the number of the question. "5 D in a Z C" stands for "Five Digits in a ZIP Code".
4 Doors Per Car
12 picture cards in a deck
15 Men on a Dead Mans Chest
12 Picture cards in a deck
9 C of H in D I? 9 circles of hell in Dante's inferno.
52 cards in a deck without the jokers
175 + 185 = 360:D
4 to 6 for dinner
1 Hump on a Dromedary
60 Minutes in a Degree
1 hole in a donut
180 degrees in a radian