To determine the better buy you have to calculate how much one package costs in each shop.
1) 6 packages cost $15.00
If you use cross multiplication you can determine how much 1 package costs:
6 packs ______$15.00
1 pack _______$x
Each package costs $2.5
2) 5 packages cost $13.25
5packs_____$13.25
1 pack______$x
Each package costs $2.65
For the second purchase each package cost $0.15 more than in the first purchase.
Is best to buy the 6 packages at $15.00