Average speed = Total distance ÷ Total time.<br><br>
&bull; Total distance = (90 × 2) + (75 × 4) = 180 + 300 = 480 km<br>
&bull; Total time = 2 + 4 = 6 h<br>
&bull; Average speed = 480 ÷ 6 = 80 km/h<br><br>
Final Answer: 80 km/h