Total time allowed = Total distance &divide; Target average speed.<br><br>
&bull; Time needed for 75 km/h average over 300 km: 300 &divide; 75 = 4 hours.<br>
&bull; Time for first 100 km at 50 km/h: 100 &divide; 50 = 2 hours.<br>
&bull; Time for next 50 km at 50 km/h: 50 &divide; 50 = 1 hour.<br>
&bull; Time used so far: 2 + 1 = 3 hours.<br>
&bull; Time remaining: 4 &minus; 3 = 1 hour.<br>
&bull; Distance remaining: 300 &minus; 100 &minus; 50 = 150 km.<br>
&bull; Required speed: 150 &divide; 1 = 150 km/h.<br><br>
Final Answer: 150 km/h