Q-7.A train 400m long is running at a speed of 72km/hr.How much time does it take to cross a telegraph post? please help fast!

Length of the train = 400 m
Speed of the train = 72 km/hr = $\frac{72\times 1000}{60\times 60} = 20 \mathrm{m}/\mathrm{s}$
So, time taken to cross a telegraph post = $\frac{\mathrm{length} \mathrm{of} \mathrm{the} \mathrm{train}}{\mathrm{speed} \mathrm{of} \mathrm{the} \mathrm{train} } = \frac{400}{20} = 20 \mathrm{s}$
Therefore, it takes 20 second to cross a telegraph post.