Two train 75 km apart approach each other on parallel tracks, each moving at 15km/h. A bird flies back and forth between the trains at 20km/h until the trains pass
each other. How far does the bird fly?

The correct solution is "37.5 km".

Explanation:

Given:

Distance between the trains,  

d = 75 km

Speed of each train,

= 15 km/h

The relative speed will be:

=

=

The speed of the bird,

V = 15 km/h

Now,

The time taken to meet will be:

hence,

The distance travelled by the bird in 2.5 h will be:

⇒

this would be about 43%