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Mr. X drove his car 20 miles to airport and then flew 400

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Mr. X drove his car 20 miles to airport and then flew 400 [#permalink] New post 20 Mar 2008, 07:10
Mr. X drove his car 20 miles to airport and then flew 400 mile at a speed 4 times faster than he drove. The entire trip took 3 hrs. What was his average flying speed in miles/ hour?
112
120
160
200
220
Please explain your answer. thanks
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Re: Mr. X drove his car [#permalink] New post 20 Mar 2008, 07:40
Capthan wrote:
Mr. X drove his car 20 miles to airport and then flew 400 mile at a speed 4 times faster than he drove. The entire trip took 3 hrs. What was his average flying speed in miles/ hour?
112
120
160
200
220
Please explain your answer. thanks


Let S = speed that he drives (4s = speed that he flew)
Time spent driving = 20/S
Time spent flying = 400/4S = 100/S

Total time = 120/S = 3 hrs
S = 40
flying spped = 4s=160 answer c
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Re: Mr. X drove his car [#permalink] New post 20 Mar 2008, 07:44
Mr. X drove his car 20 miles to airport and then flew 400 mile at a speed 4 times faster than he drove. The entire trip took 3 hrs. What was his average flying speed in miles/ hour?
Suppose driving speed is x miles per hour.
Then flying speed is 4x miles per hour.
Time taken for driving = 20/x
Time take for flying = 400/4x

Total time = 20/x + 100/x = 3
=> 120/x = 3
=> x = 40

So flying speed = 40*4 = 160

Answer C.
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Re: Mr. X drove his car [#permalink] New post 21 Mar 2008, 06:41
I donnot know for some reasons
Best answer is D
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Re: Mr. X drove his car [#permalink] New post 21 Mar 2008, 16:22
It is D

Speed formula is Speed=distance/time or time=distance/Speed

Assume x is the speed of car, and so 4x becomes speed of flight

Comparing times..

x/20+4x/400=3

x=50

Speed of flight = 4x = 200

edit: My bad... yes C it is. I used the formula wrong.
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Re: Mr. X drove his car [#permalink] New post 21 Mar 2008, 20:40
sreehari wrote:
It is D

Speed formula is Speed=distance/time or time=distance/Speed

Assume x is the speed of car, and so 4x becomes speed of flight

Comparing times..

x/20+4x/400=3

x=50

Speed of flight = 4x = 200

edit: My bad... yes C it is. I used the formula wrong.


Now I really dont get it. You said time=distance/Speed and then you go and write x/20 when it's supposed to be 20(distance) /x (speed).
Am I missing something?
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Re: Mr. X drove his car [#permalink] New post 21 Mar 2008, 21:33
moni77 wrote:
sreehari wrote:
It is D

Speed formula is Speed=distance/time or time=distance/Speed

Assume x is the speed of car, and so 4x becomes speed of flight

Comparing times..

x/20+4x/400=3

x=50

Speed of flight = 4x = 200

edit: My bad... yes C it is. I used the formula wrong.


Now I really dont get it. You said time=distance/Speed and then you go and write x/20 when it's supposed to be 20(distance) /x (speed).
Am I missing something?


that's why I said in the edit that I used the formula wrong. It's supposed to be 20/x+400/4x=3 which would lead to C. I should probably stop doing math while in commute ;-)
Re: Mr. X drove his car   [#permalink] 21 Mar 2008, 21:33
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