train and Bridge riddle
John walks over a railway-bridge. At the moment that he is just ten meters away from the middle of the bridge, he hears a train coming from behind. At that moment, the train, which travels at a speed of 90 km/h, is exactly as far away from the bridge as the bridge measures in length. Without hesitation, John rushes straight towards the train to get off the bridge. In this way, he misses the train by just four meters! If John had rushed exactly as fast in the other direction, the train would have hit him eight meters before the end of the bridge.
What is the length of the railway-bridge?
we konw ,
speed = distance/time & length of bridge = 2x
If john runs back , he misses the train by just 4 meters that means ,
that means (time taken by john to reach the right end of bridge) = (time taken by the train to reach 4meters before the start of bridge)
speed (train) = 90kmph = (2x – 4)/t & speed (john) = (x-10)/t {refer the above diagram}
t = (2x-4)/90 = (x-10)/speed(john) ………………………………(1)
similarly,
if john runs to the left of the bridge the train hits him 8 meters before the end so,
(time taken by the train to reach 8 meters before left end of the bridge ) = time taken by john to reach 8 meters before the left end of the bridge)
speed(train) = 90 = (2x + 2x -8)/t* & speed(john) = (x -8 + 10) / t*
t* = (4x-8)/90 = (x+2)/speed(john)
so,
speed(john) = (x+2)/(4x-8)(90) ……………………………………………(2)
substitute(2) in equation (1)…..
you get the answer.
NOTE: (The units does not matter as it is going to get cancelled from both RHS and LHS)
I think you did wrong
tell me where have i gone wrong..i’ll try to rectify..
units are not dealt with properly in calculation I think km/h and meters
I also got the answer 44:
Bridge is B meter long
starting position John: B/2-10
starting position train: -B
situation 1: Running back
the train traveled B-4 meter
time it took: (B-4)/vt
John traveled B/2-10 meter in the same time
John’s speed vj = vt*(B/2-10)/(B-4)
situation 2: running forward
the train traveled 2B-8 (=2*(B-4)) meter
time it took: (2B-8)/vt
John traveled (B-8) – (B/2-10) = B/2+2 meter in the same time
John’s speed vj = vt*(B/2+2)/(2B-8)
vt*(B/2-10)/(B-4) = vt*(B/2+2)/(2B-8)
(B/2-10) = (B/2+2)/2
2B-40 = B+4
B = 44
vj=vt*0,3 = 27 km/h (about 17 mi/h)
all units used are meters.
The speed isn’t used at all so its units aren’t relevant
Your Answer
More puzzles to try-
How many “red” words are in the sentence below?
First of all, I would redefine the concept of thegreen synthesis, because I don’t want to reducethis to an obvious ...Read More »Which direction is the bus going, left or right?
Power cut Riddle
Sonia is going on an extended trip for three weeks. She lives in a remote area where there are frequent ...Read More »12 dollar riddle
My goal is to have $12. If I save $1 a month then it will take me 12 months to ...Read More »Measuring Water Puzzle
You have a 12 liters jug full of water. You have two empty 8 liters and 5 liters jug. Now ...Read More »_ O _ A _ C_ Girls love it Boys use it riddle
_ O _ A _ C_ Complete this 7 letter word: Clue: 1. Girls love it 2. Boys use it ...Read More »Bus interesting puzzle
You are driving a bus. Six people get on, two people get off, then ten people get on and five ...Read More »Can you find out the number of chocolates they have right now?
A and B have a certain number of chocolates with them. If B gives one chocolate to A, they will ...Read More »How did she die?
The body of a woman is found in bed. There are no signs of violence and no marks of any ...Read More »How many members are in the Paige family?
Mr. Paige is an only child. Mr and Mrs Paige have 3 daughters and for each daughter in the family ...Read More »What strategy could they use to ensure they will go free?
There is a prison with 100 prisoners, each in separate cells with no form of contact. There is an area ...Read More »Hold water with holes riddle
It holds water even though it has many holes, when you catch, feel very soft, what is it ?Read More »Brick Optical Illusion
This is a classy optical illusion puzzle. What more you can find except bricks in the image below?Read More »Black and admired
Black he is and much admired, men seek him until they’re tired. When they find him, they break his head, ...Read More »Most Trickiest math equation riddle
Fill the question mark to correct the equation ? + ? + ? + ? + 80 + 90 = ...Read More »Which number replaces the question mark?
Placing coins on a table
Two players are playing a game. The game board is a circular table. The players have access to an ample ...Read More »Live Prisoners
There are 50 prisoners in a jail sentenced to death. One day before the execution, the jailer gives them a ...Read More »Fingers Multiplication tricky puzzle
Assume there are approximately 5,000,000,000 (5 billion) people on Earth living presently. What would you estimate to be the result, ...Read More »Find Next Number
Find Next Number in the Sequence 2 9 3 1 8 4 3 6 5 7 ?Read More »