Multiple choice

Two trains each travelling at a speed of $20$ kmph approach each other on the same straight track. A bird that can fly at a speed of $40$ kmph flies off from one train when they are $40$ km apart and heads directly for the other train. On reaching the other train it flies directly back to the first train and so forth. Before the trains crash, the total distance travelled by the bird is

  1. $20 km$
  2. $40 km$
  3. $80 km$
  4. $60 km$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The trains are 40 km apart and each travels at 20 kmph. Their relative speed is 20 + 20 = 40 kmph. The time until they crash is 40 km / 40 kmph = 1 hour. The bird flies at 40 kmph for that entire hour, so it travels 40 kmph * 1 hour = 40 km.

AI explanation

The two trains are 40 km apart and approach each other at a relative speed of 20 kmph plus 20 kmph, which equals 40 kmph. The total time taken for the trains to collide is the total distance divided by the relative speed, giving 40 divided by 40, which is 1 hour. During this hour, the bird flies continuously at a constant speed of 40 kmph. The total distance travelled by the bird is its speed multiplied by the time, which is 40 kmph times 1 hour, equalling 40 km.

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