A train of 100 meters long is running at the speed of 36 km per hour In what time it passes a bridge of 80 meters long?

Answer 1

From the moment when the train first touches the bridge, the (front end of the) train has to travel 180 meters until the back end leaves the bridge.

So how long does it take to travel 180 meters at a speed of 36 km/h?

You probably know that v = x/t. Where v is speed, x is distance and t is time.

We know v and x; we are looking for t. So bring t on one side of the equation and all other stuff on the other side (multiply by t and divide by v):

t = x/v

But we must be careful to handle the units correctly (i.e. not naively mixing meters and kilometers). Let us have a look at the speed unit.

36 km/h = 36000 m/h = 10 m/s (one hour has 60*60=3600 seconds)

t = 180m / 10m/s = 18s

So the train passes the bridge in 18 seconds.

PS:

Instead of converting v we could have converted x:

t = 0.18km / 36km/h = 0.005h

0.005 hours = 0.005*60 minutes = 0.005*60*60 seconds = 18 seconds

But I personally prefer the first way because I can do all the math in my head ;)

PPS:

Keep learning and try to solve such puzzles by yourself. Try hard!