Case

Brainteaser: Birds on train

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Problemdefinition

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?


Kommentare

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.


Kurzlösung

The bird flies a distance of 90 km back and forth until the trains reach each other.


Detaillierte Lösung

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.

Schwierige Fragen

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Beste Antwort bisher:
Ex BCG | MBB Specialist | #1 Expert for meetings done (1000+) and recommendation rate (100%)

Hi Anonymous, there may be several reasons for that; some of them, in order of frequency, are: Your structure is not complete enough – you are just using not MECE/ not complete structure... (mehr)

Hey, I am new to this too, but I think I have some ideas for your problem. Your approach looks good, but I think you have a mistake in how you time the trains. You say that there is one train ev... (mehr)

Bisher beste Antwort von 3 Antworten:
Sibren
Experte
Two years conulting experience, went through lots of interviews and coached several candidates to MBB offers

Hi, good question! Probably somewhat depending on your interviewer, but this is my view on it. In this situationit's important to try to showcase as mluch of your consulting skills as possible. Be t... (mehr)

Bisher beste Antwort von 2 Antworten:
Marco
Experte
Specific coaching: company you are applying for; 1st or 2nd round; your professional background; ...

Hi, here's how I would approach it - Define the geographic area to be covered [the whole world? the U.S.? NYC? Manhattan?...for this example I'll use Manhattan] - Population living there [~ 1... (mehr)

Bisher beste Antwort von 3 Antworten:
McKinsey / Accenture / More than 300 real MBB cases / Collected all Big 3 offers / Harvard Business School

Hi, First of all, I totally agree with Francesco on the process. You should always come back to the cases you've solved and reevaluate your structures. Secondly, I believe it's very much related... (mehr)

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