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?
First, we need to find the difference between the two averages: 3.2 miles/week - 2.8 miles/week = <<3.2-2.8=0.4>>0.4 miles/week
Then we divide the difference by the original average and multiply by 100% to find the percent decrease: 0.4 miles/week / 3.2 miles/week * 100% = 0.125 * 100% = 12.5%. Answer: \boxed{12.5}.