Brain Teasers - Coins

Solved 600+ times
4.6 5 235
Difficulty: Intermediate | Style: Brain Teaser | Topics: Brain teaser

Problem Definition


Imagine, we are sitting together at a perfectly round table. We both have an infinite pile of quarters. These are the rules of the game: “We'll take turns putting one quarter down on the table, no overlapping allowed, and the quarters must rest flat on the table surface. The first guy who can't put a quarter down loses."

Who should start in order to win, why and how does your strategy work?



There are 8 marbles that weigh 1 ounce each, and 1 marble that weighs 1.1 ounces. The marbles are all uniform in size, appearance, and shape. You have a balance that contains 2 trays. You are only able to use the scale 2 times.


How do you determine which marble is the heaviest using only the scale and marbles in 2 weighings.


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

Detailed Solution


You have to start. You place a quarter exactly in the center of the table. After that, whenever I put quarter on the table, mimic my placement on the opposite side of the table. If I have a place to place a quarter, so will you. I will run out of places to put a quarter before you do.


Place 3 marbles on each tray.

  • If the first weighing doesn't balance, remove all the marbles from the lighter side, and place one marble on each tray from the heavier tray. The heavier side is the 1.1 ounce marble, but if they balance, then the marble from the heavier tray from the first weighing that was not weighed in the second weighing is the heavier one.
  • If the marbles balance on the first weighing, remove the marbles from the trays, and place 2 of the remaining not weighted marbles on the trays, one on each tray. If one is heavier, it is the heavier marble, but if they balance, the remaining not weighted marble is the heavier one.

Difficult Questions

Related consulting question(s)

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... (more)

Best answer so far:
Ex BCG | MBB Specialist | #1 Expert for meetings done (900+) 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... (more)

Best answer so far out of 2 answers:
Interview specific coaching: company you are applying to; 1st or 2nd round; your professional background and more

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... (more)

Best answer so far out of 3 answers:
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... (more)

Hey! The cases usually test strategy, finance and business development. You should be familiar with the common business concepts and financial terms and calculations (CF, BS etc). Abo... (more)

Related BootCamp article(s)

Typical Question Types in Written Tests

Questions among all tests can be categorized in math, GMAT-style, chart reading and brain teaser questions. Each type needs to be approached differently.

4 Comment(s)

Brain Teasers

Learn how to tackle brain teasers and what purpose they serve in consulting case interviews

1 Comment(s)
4.6 (235 ratings)
4.6 5 235
Case exhibits

No case exhibits yet