A train running at 72 km/h crosses an electric pole in 9 seconds and a bridge in 29 seconds. The length of the bridge (in metres) is:

72 km/$h=72\times 5/18=20$ m/s, so the train itself is $20\times 9=180$ m long. Crossing the bridge it covers $20\times 29=580$ m, which is bridge + train, so the bridge is 580 − 180 = 400 m. (580 m is the whole distance covered — the train's own length still has to be taken off.)