Answer:

Distance covered in half an hour at 30km/hr = s×t = 30×1/2=15km.

Distance coveted in 1hr at 25km/hr= s×t= 25×1 =25km.

Distance covered in 2 hrs at 40km/hr=s×t= 40×2=80km

Therefore, total distance covered= 15+25+80=120km

Total time taken= 3hrs and 30 mins=7/2 hrs.

Therefore, average speed=total distance covered/total time taken = 120×2/7 = 34.28km/hr

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Explanation:

Distance covered in half an hour at 30km/hr = s×t = 30×1/2=15km.

Distance coveted in 1hr at 25km/hr= s×t= 25×1 =25km.

Distance covered in 2 hrs at 40km/hr=s×t= 40×2=80km

Therefore, total distance covered= 15+25+80=120km

Total time taken= 3hrs and 30 mins=7/2 hrs.

Therefore, average speed=total distance covered/total time taken = 120×2/7 = 34.28km/hr