Brainteaser: Birds on train

Problem Definition

Assume we have two trains moving on the same track. These trains are approaching each other head-on meaning that Train A is moving towards Train B & vice versa. The track these trains are on has a distance of 120 km. Train A has a speed of 26.6 km/hour and Train B has a speed of 19.7 km/hour.

Now on top of Train A, there is a little bird that is flying with a speed of 33.4 km/hour. The bird flies from Train A to Train B and back. He keeps doing this until both trains approach each other.

The question is: How much distance has the bird flown once both trains approached each other?

Comments

Brain Teasers are short questions with difficult logical or creative answers. They are a good way to test your analytical skills and capacity to think out of the box. These short questions also show the interviewer your ability to elaborate on difficult tasks from the real world. Each Brain Teaser is supposed to be solved by the interviewee in about 5-10 minutes.

If the interviewee gets stuck in a brain teaser (i.e. spends more than 10 minutes), propose to go on to the next one.

Short Solution

Solution

Step 1: Sum up the speed of both trains and divide the total distance of 120 km by the summed up speed.

26.6+19.7=46.3 km/hour.

120 km/46.3 km=2.6 hours which is about 2 hours and 40 minutes. Feel free to round up and estimate here.

Step 2: Now that we know how long it will take before the two trains reach each other, we can calculate the distance the little bird has flown.

2.6 hours*33.4 km=87 km.

Conclusion: In the timeframe of 2.6 hours the bird flies a distance of 87 km back and forth until the trains reach each other.