A train completely crosses a bridge of length 80 m in 15 s. Find the length of the train if its speed is 72 km/h.

Let's assume the length of the train is 'y' m.

A train completely crosses a bridge of length 80 m in 15 s.

Length of bridge = 80 m

time taken to cross the bridge = 15 s

speed of train = 72 km/h

= $$72\times\ \frac{1000}{3600}$$ m/s     [we know that 1km = 1000m and 1hour = 3600 seconds.]

= 20 m/s

speed of train = $$\frac{length\ of\ train+length\ of\ bridge}{time}$$

$$20=\frac{y+80}{15}$$

y+80 = 300

y = 300-80

length of the train = y = 220 m