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time and speed : prob already discussed but want to discuss

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Senior Manager
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time and speed : prob already discussed but want to discuss [#permalink] New post 28 Aug 2010, 22:51
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The ‘moving walkway’ is a 300-foot long walkway consisting of a conveyor belt that moves continuously at 3 feet per second. When Bill steps on the walkway, a group of people that are also on the walkway stands 120 feet in front of him. He walks toward the group at a rate of 3 feet per second. Once Bill reaches the group of people, he stops walking and stands with them until the walkway ends. What is Bill’s average rate of movement for his trip along the moving walkway?
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Re: time and speed : prob already discussed but want to discuss [#permalink] New post 29 Aug 2010, 08:27
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By the time Bill catches up with the people standing at the walkway, this is the summary of his trip :

240 feet - 40 sec - 6feet/sec (240 feet wrt to ground = 120 feet relative to people standing on walkway)

For the next 60 feet ( that is what is left to cover);

60 feet - 20 sec - 3 feet/sec

So, average speed = (240+60)/(40+20) = 5 feet/sec.
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Re: time and speed : prob already discussed but want to discuss [#permalink] New post 29 Aug 2010, 08:30
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I would follow the following steps to solve this problem:

1. Determine the amount of time it takes Bill to reach the group based on the difference in speed between Bill and the group (difference in speed is 3ft/s)
=> 120 ft / (6-3)ft/s = 40s

2. Determine the distance Bill travelled at a speed of 6ft/s and 3ft/s, respectively
=> 40s * 6ft/s = 240ft => Bill travelled 60ft at the lower speed of 3ft/s, which took him 60ft / 3ft/s = 20s

3. Determine Bill's average speed by dividing total distance by total time for the trip
=> 300 ft / (40s + 20s) = 5ft/s

Hence, the average total speed is 5ft/s.
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Re: time and speed : prob already discussed but want to discuss [#permalink] New post 29 Aug 2010, 09:26
thanx ..guys

I guess I had temporary sleep of mind ...
Re: time and speed : prob already discussed but want to discuss   [#permalink] 29 Aug 2010, 09:26
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time and speed : prob already discussed but want to discuss

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