Answer:
Michelle purchased 25 audio files in January. The percent increase is 60%
Solution: 
Given Data: 
Michelle purchases = 25 audio files in January 
Again purchased = 40 audio files in February 
We need to find the percent increase for the given data. 
Step 1: 
Use the formula for percent change ,

Step 2: 
First we find the amount of change from the question. 
Amount of change = 40 – 25 
By subtracting, we get 15. 
Step 3: 
substitute the values in formula we get,
Formula percent change = 
 = 0.6
 = 0.6 
Percent = 0.6 
 100 = 60 %.
 100 = 60 %. 
Result: 
Michelle purchased 25 audio files in January. And In February she purchased 40 audio files. Hence the percent increase is 60%.