# Thread: Find average velocity without a given time or distance??

1. ## Find average velocity without a given time or distance??

A family travels from hither to yon one afternoon. On this day, the driver spends half the distance traveling at 77 km/hr and the remaining half at 50 km/hr. What is the average speed of the car during the trip?

To work the problem, I assumed the trip took 2 hrs, then found how much time was spent at each speed, then found a weighted average of 63.5 km/hr. This is incorrect.

Someone told me I can't assume the time it took to make the trip. Why not? Half is half, and the proportions will be the same..

How do I approach the problem?

2. ## Re: Find average velocity without a given time or distance??

Originally Posted by OneMileCrash
A family travels from hither to yon one afternoon. On this day, the driver spends half the distance traveling at 77 km/hr and the remaining half at 50 km/hr. What is the average speed of the car during the trip?

To work the problem, I assumed the trip took 2 hrs, then found how much time was spent at each speed, then found a weighted average of 63.5 km/hr. This is incorrect.

Someone told me I can't assume the time it took to make the trip. Why not? Half is half, and the proportions will be the same..

How do I approach the problem?
(speed)(time) = distance, time = (distance)/(speed)

distance d/2 covered at 77 km/hr ... time = (d/2)/77

distance d/2 covered at 50 km/hr ... time = (d/2)/50

total time = d/154 + d/100

average speed = (total distance)/(total time) = d/(d/154 + d/100) = 1/(1/154 + 1/100) = 7700/127 km/hr

3. ## Re: Find average velocity without a given time or distance??

Gotcha, thanks!

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# how to find average speed without distance and time

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