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# Fly

25 replies to this topic

### #1 rookie1ja

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Posted 30 March 2007 - 03:57 PM

A Fly - Back to the Cool Math Games
Two trains, 200 km apart, are moving toward each other at the speed of 50 km/hour each. A fly takes off from one train flying straight toward the other at the speed of 75 km/hour. Having reached the other train, the fly bounces off it and flies back to the first train. The fly repeats the trip until the trains collide and the bug is squashed.
What distance has the fly traveled until its death?
(There is a complicated and an easy way to solve this math brain teaser.)

This old topic is locked since it was answered many times. You can check solution in the Spoiler below.
Pls visit New Puzzles section to see always fresh brain teasers.

Spoiler for Solution

Spoiler for old wording

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### #2 larryhl

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Posted 05 June 2007 - 07:11 PM

good type of question to know for interviews =)

the sequence takes too long. just realize the fly will only be able to fly for 2 hours and multiply by speed to get distance.
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### #3 lukegaru

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Posted 03 July 2007 - 08:21 PM

150klm
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### #4 Slick_Rick9009

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Posted 26 July 2007 - 09:39 AM

I just want to say that I am very proud of this specific question because it is the only one I've seen that doesn't have the same questions and the same wrong answers being repeated a hundred times. Wrong answers are fine. But if you read and find out that they were already proven wrong, then you wouldn't be posting it. And if you're going to post something, shouldn't you read the previous posts to make sure you're not repeating? That happens a lot and this is the first I've seen that it hasn't happened. I'ts made a nerd very happy.
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### #5 captainobvious00

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Posted 07 August 2007 - 08:10 AM

what u have to take into consideration is that when the fly hits the other train, it will die. it would have to slow down first. then when it goes back, it will have to speed back up, therefore causing less travel time/distance
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### #6 twilly

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Posted 14 August 2007 - 04:31 AM

i have one question, how can a fly go faster than a train?
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### #7 mdsl

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Posted 14 August 2007 - 08:43 PM

i have one question, how can a fly go faster than a train?

Easy, if the fly is inside the train and flys in a relative direction equal to the direction of the train, the fly will be going faster than the train relative to the tracks.

As it applies in this question, i don't know. It's just a puzzle.
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### #8 tydis

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Posted 18 August 2007 - 11:44 PM

ok this might be a stupid question but how do you know it takes 2 hours? the puzzle never says so.
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### #9 mdsl

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Posted 20 August 2007 - 12:35 PM

ok this might be a stupid question but how do you know it takes 2 hours? the puzzle never says so.

Two trains 200 km from each other are moving at the speed of 50 km/hour towards each other.

Each train is going 50 km/hour, they are traveling in opposite directions towards each other, and they are initially 200 km apart. The distance between the trains is decreasing at a rate equal to the sum of the speed of each train, or 100 km/hour.

It will take 2 hours to close the gap.
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### #10 Dreaken667

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Posted 24 August 2007 - 01:59 AM

I'd like to point out that there is a minor detail which has not been considered. Assuming that the fly begins his suicidal flight at the precise time that the trains are at their original points, then the calculation presented would be correct. However, the puzzle, as stated, does not in fact say when the fly begins his spiraling death. If he were to start flying half way through the trip for example, then he would only travel 75km. Therefore, unless the puzzle is reworded, the answer would rely on the point in which the fly has his nervous break down and decides that existence is futile.
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