the ratio of cost to bottles is the same

9/5 = X/12

cross multiply

5X =9(12) = 108

X = 108/5 = $21.60 for 12 bottles

12 is over twice 5, so the cost should be over twice as much or more than $18

12 = 2.4(5) so multiply $9 by 2.4 to get $21.60 for 12 bottles