uhm...
I think you would save 7.5$
you would do 16-6 =10
then
10 divided by 4= 2.5
then you would do 2.5 times 3=7.5
10 dollars less, then
so THE ANSWER would be 7.5$ difference, because there are 4 weeks in a month and it only lasts Zach 3 weeks.
*sorry for all the random useless stuff*