Given:Length of Train (L) = 120 meterSpeed of train (s) = 54 km/hr = 54 × \(\frac{5}{18}\) m/sec = 15 m/secTime taken to cross the bridge (t) = 20 secondsFormula used:Speed of train = \(\frac{(Length\ of\ Train)\ +\ (Length\ of \ Bridge)}{Time\space taken\space to\space cross\space the\space bridge}\)Calculation:Let us assume length of bridge is "y".Then,\(\frac{(120\ +\ y)}{20}\) = 15120 + y = 20 × 15 120 + y = 300y = 180Length of the bridge = 180 mHence, the correct answer is "180".Additional InformationTo convert km/hr scale to the m/s scale we multiply the value of km/hr to \(\frac{5}{18}\).