A 4-pack of identical batteries costs 5.20 dollars and a 10-pack costs 12.50 dollars. Enter the savings per battery from choosing the better pack, in dollars.

Explanation

The smaller pack costs 1.30 dollars per battery and the larger costs 1.25 dollars, so the saving is 0.05 dollars per battery. Comparing only 5.20 with 12.50 would hide the different battery counts and favor the wrong pack.