You have to divide total price by "18 times" for the first and by "12 times" for the second.
So,
The 18-issue subscription costs [tex] \frac{40,50}{18} =\$2.25[/tex] per issue.
The 12-issue subscription costs [tex] \frac{33,60}{12} =\$2.80[/tex] per issue.
Therefore the 12-issue subscription costs more per issue.