From the graph, find the runner's resting heart rate (at 0 km) and the heart rate at the 30 km mark.<br><br>
Heart rate reserve (HRR) = 200 - resting heart rate.<br>
Heart rate above resting at 30 km = HR at 30 km - resting heart rate.<br>
Percentage of HRR used = (HR above resting / HRR) × 100%.<br><br>
Final Answer: Read from the graph and compute.