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It takes a train 3 seconds to completely pass a single point. At the same speed, it fully clears a 270-meter-long tunnel in 12 seconds. How long is the train?

I apologize, I posted this once before and just got a single answer. Would someone mind explaining how they got it too? Thanks!

Les40 gave the answer to this one as 90 metres.

The train is x metres long:
If the train takes 3 seconds to pass a single point the speed of the train is x/3 metres per second.
The train fully clearing a 270 metre tunnel in 12 seconds will have to travel 270 + x metres in 12 seconds i.e. at a speed of (270+x)/12 metres per second.
These two speeds are the same
Therefore:
x/3 = (270+x)/12
4x = 270 + x
3x = 270
x=90
The train is 90 metres long.

I think!