Answer:
The average speed required during the rest of the trip is 81 Km/h
Step-by-step explanation:
It is 81 Km/h because we already know the average speed of 74 km/h during the 2 hours trip. Then the distance in total should be 2*74 km, then if we notice that it is undergone with two different speeds we can make the next calculation: (Speed1 *1/3 + Speed2*5/3)=(74*2), here 1/3h corresponds to the 20 minutes at 39 km/h and the 5/3h to the 100 minutes left, both in hours, since the speed is given also in that unit of time, i.e., the speed is given in km/h. We already know Speed1=39 km/h, then Speed2*5/3=74*2-Speed1*1/3=(148-13)km, then Speed2= 135 Km/(5/3 h)=135*3 km /5 hs=81 km/h. So the average speed required afterwards to get a total average of 74km/h during the whole trip is 81 km /h.