Maths

[Legolassie]

Hi all you maths wizards, please can you solve this for me?
A train with a length of 100 metres takes 30 seconds to travel over a 400 metre bridge. Assuming thatthe train's speed is constant, how many kilometres per hour is it travelling at? I would appreciate any help. Thanks in anticipation.

[BigDave44]

It travels (400 + 100 = ) 500 metres in 30 seconds.
(500 x 120) / 1000 kilometeres in one hour = 60kph

[Buenchico]

The train won't be totally clear of the bridge until the engine has travelled 500m. (i.e. the engine has to cross the 400m bridge and then go a further 100m to get the last coach clear of the bridge).

500m in 30 seconds would mean 1km in 60 seconds and 60km in one hour.

So, 60 km/h.

Chris

PS: I've assumed that the 'time taken to cross' means the time that it takes to get the whole train clear of the bridge (which is what I suspect the questioner wants). However, the 'scientific' approach is to to say that if any part of the train takes 30 seconds to cross the bridge, thats 0.4km in 30 secs = 0.8km in 1 minute = 48km/h.
Since the question doesn't define exactly what is meant by 'cross' either interpretation is possible. (I've assumed that the information about the length of the train is meant to be relevant. If so, my first answer is correct. However, if the information is a 'red herring', the second answer is correct!)

[Legolassie]

Thanks a bunch. I had fogotten to add the extra 100 to clear the bridge. Very much appreciated. Thanks a lot.