Total distance = 80 km.<br>
Total time = 87 minutes = \( \frac{87}{60} \) hours.<br><br>
Average speed = Total distance &divide; Total time<br>
= 80 &divide; \( \frac{87}{60} \)<br>
= 80 &times; \( \frac{60}{87} \)<br>
= \( \frac{4800}{87} \)<br>
= 55.172...<br>
&asymp; 55.2 km/h (to 1 decimal place).<br><br>
Final Answer: 55.2 km/h