A cat travels across gardens at 240 metres an hour and returns on the same route at 160 metres per hour -- what is his average speed?

Remember the basic equation of velocity:



Here we want to find the value for velocity, so rearrange:



So we need now to find the distnace traveled, and the time it takes. Let's call the width of the garden W. Then the total distance traveled is 2*W, so that's the numerator. Now you need to figure out how long the total trip took to complete - which is the sum of the time for the trip over plus the trip back. The time for each segment of the cat's journey is found using the formula time = distance/velocity; do you see how I got that? So the trip over takes W/240, and the trip back took W/160, and the total time is W/240 + W/160. Now you can find the average speed:



See how the W terms cancel out? Can you take it from here, and post back the final answer that you get?

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Originally Posted by ebaines

Remember the basic equation of velocity:



Here we want to find the value for velocity, so rearrange:



So we need now to find the distnace traveled, and the time it takes. Let's call the width of the garden W. Then the total distance traveled is 2*W, so that's the numerator. Now you need to figure out how long the total trip took to complete - which is the sum of the time for the trip over plus the trip back. The time for each segment of the cat's journey is found using the formula time = distance/velocity; do you see how I got that? So the trip over takes W/240, and the trip back took W/160, and the total time is W/240 + W/160. Now you can find the average speed:



See how the W terms cancel out? Can you take it from here, and post back the final answer that you get?

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The average speed is 200 meters I think?

Nope, try again.

Simplify the fraction just below there;

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Originally Posted by ConfusedAutumn

The average speed is 200 meters I think?

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Sorry - that's incorect. You can't just average the two speeds. You can see why that doesn't work if you consider an easier case. Suppose you go 60 miles at 60 MPH, taking 1 hour of time. Then you cut your speed to 1 MPH and go another 60 miles - that will take another 60 hours, right? Total elapsed time is 1 hour + 60 hour= 61 hours, and total distance traveled is 2*60 = 120 miles. So your average speed is 120 miles/61 hours = 1.97 MPH. Compare that to simply averaging the speeds: (60+1)/2 = 30.5 MPH. Clearly averaging the speeds gets you the wrong answer.

Quote:

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Originally Posted by ebaines

Sorry - that's incorect. You can't just average the two speeds. You can see why that doesn't work if you consider an easier case. Suppose you go 60 miles at 60 MPH, taking 1 hour of time. Then you cut your speed to 1 MPH and go another 60 miles - that will take another 60 hours, right? Total elapsed time is 1 hour + 60 hour= 61 hours, and total distance traveled is 2*60 = 120 miles. So your average speed is 120 miles/61 hours = 1.97 MPH. Compare that to simply averaging the speeds: (60+1)/2 = 30.5 MPH. Clearly averaging the speeds gets you the wrong answer.

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Ok thank you, I think I understand it now, is the answer 192?

Ah! That's it! Well done! :)

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Originally Posted by Unknown008

Ah! That's it! Well done! :)

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Thank you for all your help :D xx