a healthy young student used an elliptical trainer for 40 minutes at the gym. although the workload remained constant, the student's heart rate rose to 150 beats per minute during the first 5 minutes, stayed at that level for the next 25 minutes, and then started to rise. it was 180 beats per minute at the end of the exercise period. what was the time interval between qrs complexes when the student's heart rate was 180 beats per minute?