Solution to: Train Trouble

Let the length of the bridge be x meters.

Running towards the train, Charles covers 0.5x - 10 meters in the time that the train travels x - 4 meters. Running away from the train, Charles covers 0.5x + 2 meters in the time that the train travels 2x - 8 meters.

Because their speeds are constant, the following holds:

(0.5x - 10) / (x - 4) = (0.5x + 2) / (2x - 8)

which can be rewritten to

0.5x² - 24x + 88 = 0

Using the quadratic formula we find that x = 44, so the railway-bridge has a length of 44 meters.

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