ABC Electronics has the better deal.
Here's what I did.
In order to compare the prices, you have to find the price of just one battery for each of the companies. This is like a better buy problem, and for these, we calculate unit price. The unit price has a formula... Unit Price= /frac{Price}{Quantity}
ABC Electronics: /frac{4.8}{8}= 0.6 => $0.60 per battery
Better Batteries: /frac{10.2}{15}= 0.68 => $0.68 per battery
ABC Electronics has the lower cost for one battery. If you calculate the both prices for the same quantity, then you can just see which one has the lower cost.
Now answering your second question of the difference in cost per batter. In order to find the difference, you have to subtract the lower price from the greater price. So...
0.68-0.60=0.08 There is a difference of $0.08!
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Which store has the better buy? ABC Electronics!
What is the difference in the cost per battery? $0.08
I hope that helped. Please tell me if I need to explain more clearly in the comments below. :D