Determine the average speed for the car trip

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Homework Help Overview

The problem involves calculating the average speed of a car trip with varying speeds over different segments of time. The original poster outlines the travel times and speeds between cities, along with a break period.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss calculating distances for each segment based on time and speed, with some questioning the method of averaging speeds directly. There is confusion regarding the total distance and total time calculations.

Discussion Status

Participants are actively engaging in clarifying the calculation process. Some have provided guidance on how to compute distances for each segment and the importance of including break time in total time calculations. There is no explicit consensus on the correct approach yet, as participants are exploring different interpretations of the problem.

Contextual Notes

Participants note the need to convert time from minutes to hours for average speed calculations, and there is an emphasis on maintaining units throughout the calculations. The original poster's understanding of average speed appears to be a point of confusion.

toothpick09
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Homework Statement


A person travels by car from one city to another with different constant speeds between pairs of cities. She drives for 15.0 min at 65.0 km/h, 20.0 min at 75.0 km/h and 60.0 min at 40.0 km/h, and spends 15.0 min eating lunch and buying gas.

Determine the average speed for the trip.


Homework Equations


Average Speed = total distance/total time


The Attempt at a Solution


(65+75+80)/(15+20+60+15)=1.64 which isn't logical. I don't understand how to compute the total distance?
 
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Well, the trip is naturally broken up into segments. So don't worry about calculating the total distance if that confuses you. Calculate the distance traveled in each segment and then add them up. For example, first she travels for 15 min at 65 km/h. Okay, how far did she travel in the 15 minutes?
 
I took each time divided by 60 to give me hours then multiplied that number by the km/h. Added them up and got 81.25. What am I doing wrong?
 
toothpick09 said:
I took each time divided by 60 to give me hours then multiplied that number by the km/h. Added them up and got 81.25. What am I doing wrong?

What are the units? If you included the units you would see that this is not an average speed. What you got is the total distance. To find the average speed, you must still divide by the total time (in hours if you want the average speed in km/hr). Don't forget to include the 15 minutes break time.
 
So after getting 81.25 which would be my total distance I divide by the total time which would be 110. This gives me .738636. This doesn't seem logical but is what I am doing correct?
 
You really would make your life a lot easier if you just carry the units in your calculations.

The total distance traveled is 81.25km as you said

The total time taken for the trip is 110min just as you said

So you have \frac{81.25}{110} km/min

Can you convert that to km/h ?
 

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