Kyle2514 Posts: 1, Reputation: 1 New Member #1 Feb 25, 2013, 09:46 AM
Average speed
A man drives at 20mph and comes home over the same distance at80mph. The mountains are 320 miles away. What is his average speed?
 ilovecupcakes Posts: 3, Reputation: 1 New Member #2 Feb 25, 2013, 02:30 PM
I believe its 0.1875 because your calculating its time so T=d/s which means time =distance divide by its speed. :D
 ebaines Posts: 12,132, Reputation: 1307 Expert #3 Feb 26, 2013, 10:44 AM
Originally Posted by ilovecupcakes
i believe its 0.1875 because your calculating its time so T=d/s which means time =distance divide by its speed. :D
Not even close. Clearly the average speed is going to be somewheer between the two speeds used.

Kyle2514: remember that average speed = total distance traveled divided by total time for the trip. To get started you need to add together the time taken to (a) drive 320 miles to the mountains at 20 MPH and (b) drive 320 miles back at 80 MPH. Show us your attempt at getting the answer and we'll check it for you.
 make Posts: 4, Reputation: 1 New Member #4 Feb 27, 2013, 12:12 PM
Wouldn't it be 50mph? (20+80)/2=50 He went the same distance both ways so surely the distance wouldn't matter?
 ebaines Posts: 12,132, Reputation: 1307 Expert #5 Feb 27, 2013, 03:15 PM
No - you can't average the speeds to get the average speed, even though the distance covered in th etwo legs is the same. If the average was 50 MPH then the time to travel the 640 miles would be 640 miles/50 MPH = 12.8 hours. But if you calculate the actual time taken it's 320 miles/20 MPH + 320 miles/80 MPH = 16 + 4 = 20 hours. So 50 MPH is incorrect.

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