Average speed = Total distance &divide; Total time (including dwells).<br><br>
&bull; Segment 1: 20 km &divide; 60 km/h = \( \frac{1}{3} \) h = 20 min travel + 20 min dwell = 40 min.<br>
&bull; Segment 2: 20 km &divide; 30 km/h = \( \frac{2}{3} \) h = 40 min travel + 20 min dwell = 60 min.<br>
&bull; Segment 3: 20 km &divide; 20 km/h = 1 h = 60 min travel + 20 min dwell = 80 min.<br>
&bull; Total time: 40 + 60 + 80 = 180 min = 3 hours.<br>
&bull; Total distance: 60 km.<br>
&bull; Average speed: 60 &divide; 3 = 20 km/h.<br><br>
Final Answer: 20 km/h