Final answer:
To average 80 km/hr for the entire trip, the truck must go at a speed of 80 km/hr during the second half of the trip.
Step-by-step explanation:
To find out how fast the truck must go during the second half of the trip to average 80 km/hr for the entire trip, we can use the concept of average speed. Average speed is calculated by dividing the total distance traveled by the total time taken.
Let's assume the entire trip is made up of two equal halves. In the first half, the truck travels at a speed of 70 km/hr. So, if we consider the distance traveled in the first half to be 'd', the distance traveled in the second half will also be 'd'.
Therefore, the total distance traveled for the entire trip is 2d. Now, to average 80 km/hr for the entire trip, we can use the formula:Average speed = Total distance / Total time
80 km/hr = 2d / Total time
Since the time taken for the first half is the same as the time taken for the second half, we can represent the total time as '2t', where 't' is the time taken for each half.
Substituting the values, we have:80 km/hr = 2d / (2t)
Cancelling out the common factors, we get:80 = d / t
Since the distance traveled in the first half is 'd' and the time taken for the first half is 't', the speed during the second half must be '80' km/hr to average 80 km/hr for the entire trip.