Answer:
7.4%
Explanation:
The cost decreased by 3.40 - 3.15 = 0.25
The percentage decrease can be found by using the formula shown below:
Percentage Decrease =

Where
"New" is the newest, latest price (3.15)
"Old" is the first price (3.40)
Now we find it:

So percentage decrease is 7.4% (rounded to nearest tenth)