A train is 100 m long and is moving with a speed of 72 kmph. The time taken by the train to cross a bridge of length 0.5 km is

length of the bridge $=0.5\text{km}=0.5\times 1000=500\text{m}$

total distance $=100\text{m}+500\text{m}=600\text{m}$

average speed $=\text{72}\text{kmph}=\text{72×}\frac{5}{18}=20{\text{ms}}^{-1}$

time taken  $=\frac{\text{Total}\text{distance}}{\text{Average}\text{Speed}}$

$=\frac{600}{20}=30\text{s}$