A runner's pace, in minutes per mile, during a 60-minute training run is described as follows: For the first 15 minutes, the pace steadily decreases from 10 minutes/mile to 6 minutes/mile. For the next 20 minutes, the pace remains constant at 6 minutes/mile. For the following 10 minutes, the pace steadily increases from 6 minutes/mile to 8 minutes/mile. For the last 15 minutes, the pace steadily increases from 8 minutes/mile to 12 minutes/mile. During which of the following periods was the runner's speed increasing?