Ok. For any price higher than $80, it will be cheaper for Jay to buy on line. Below $80, it will be cheaper to buy it at the store (assuming that there are no sale taxes)
This is why:
15%/100%= 12/x, x is the price of the car stereo.
(15)(x)=(12)(100)
15x= 1200
x=80 at this price it does matter whether he buys it on line or at the store. Because 15% discount on an item that costs $80 is $12. [ (.15)(80)=12]. He saves 12 dollars but he has to spend 12 dollars in shipping. So, he end up paying $80.. However, if the price of the item is more than 80, it will be cheaper for him to buy on line.