Two trains , 100 km apart , travel towards each other on the same track . One train travels at 40 km/hr, the other travels at 60 km/hr . A bird starts flying at a speed of 90 km/hr , at the same location of the faster train . When it reaches the slower train , it turns around and flies in the opposite direction at the same speed . when it reaches the faster train , again it turns around and so on . When the two trains collide , how far has the bird flown ?

Options

A. 90 km
B. 45 km
C. 180 km
D. 135km

Answer

A

Explanation

Since the trains are 100 km apart , and the trains are travelling towards each other at 40 and 60 km/hr , the trains will collide in one hour . The bird must be flying at 90 km/hr during this time , so the bird travels 90 km .