By definition, the average speed is:
vav=Ttotltot where ltot is the total distance covered by the train in time Ttot.
The distance covered during the first part of the journey is 50 km, during the second part is 0 km and during the third part 90km/h⋅2h=180 km. Thus:
ltot=50km+0km+180km=230km The time spent on the first part of the journey is 0.5h, on the second part 20/60=1/3 h and on the third part 2h, The total time is:
Ttot=0.5h+1/3 h+2h=17/6 h The average speed wil be:
vav=Ttotltot=17/6 h230km≈81.2 km/h Answer. 81.2 km/h.
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