According to her running log, Shivani averaged 3.2 miles per week last month and 2.8 miles per week this month. By what percent has her average decreased?
%
To find the percent decrease, we need to find the difference between the two averages and divide by the original average.
The decrease from last month to this month is 3.2 - 2.8 = 0.4 miles per week.
The original average was 3.2 miles per week.
The percent decrease is (0.4 / 3.2) * 100% = 12.5%. Answer: \boxed{12.5}.