Coin Flipped 3 times interview question

If you flip 3 coins and I tell you that you got at least one heads, what's the probability that all three came up heads?

I came up with 2 different answers to this question depending how you interrupt knowing that one is a heads.

34 Comments
 
Best Response

I got 1/7. For those of you interested in the detailed calculation: Let P(A) = Probability of getting 3 heads Let P(B) = Probability of getting at least 1 head or conversely 1 - probability of getting no heads. (1-(1/2)^3)

P(A) = (1/2)^3 = 1/8 P(B) = (3c1)(1/2)^3 + (3c2)(1/2)^3 + (3c3)(1/2)^3 = 7/8 by binomial theorem

Bayes: P(A|B) = P(B|A)*P(A)/P(B) P(B|A) = 1 (given you have 3 heads, you have a 100% chance of getting at least 1 head P(A) = 1/8 P(B) = 7/8

P(A|B) = P(B|A)*P(A)/P(B) = 1 * (1/8) / (7/8) = 1/7 Boom.

"Luck is what happens when preparation meets opportunity"
 

The correct answer is 0%. Your strong attraction to all forms of currency would cause you to grab the coin before it lands, investing it in a diverse portfolio which generates a 20% IRR.

 

We did 3 flips, let's call them flip 1, filp 2, and flip 3.

There are two possible interpretations of 'you got at least one heads' I think of:

  • The interrogator decided to tell me the result of one of the flips. It hapened to be flip x which hapens to be heads. -> Two remaining flips need to be heads in order to have 3 heads, both of them are independent of x, which mean we have a probability of 1/4 of having 3 heads: 1/2 for the first remaining flip multiplied by 1/2 for the second one.

  • The interrogator tells me 'you got at least one heads' as a genuine description of what he is seeing, so that I know which are the possible states. -> For each flip there are 2 outcomes, meaning there is a total of 8 (2 x 2 x 2) outcomes. The assertion of the interogator only means that one of outcomes is not possible (flip 1 is not head, flip 2 is not head, flip 3 is not head), there are 7 remaining possible outcomes and only one the 7 is (flip 1 is head, flip 2 is head, flip 3 is head). The probability of having 3 heads is thus 1/7.

If you want the interrogator to be clear you can have the following discussion with him: - Which flip are you telling me about? - The second one. (for example, it doesn't really matter). - Would you had told me about the outcome of the second flip even if it was not heads?

Yes -> 1/4 No -> 1/7

 
"leolum" We did 3 flips, let's call them flip 1, filp 2, and flip 3.

There are two possible interpretations of 'you got at least one heads' I think of: - The interrogator decided to tell me the result of one of the flips. It hapened to be flip x which hapens to be heads. -> Two remaining flips need to be heads in order to have 3 heads, both of them are independent of x, which mean we have a probability of 1/4 of having 3 heads: 1/2 for the first remaining flip multiplied by 1/2 for the second one. - The interrogator tells me 'you got at least one heads' as a genuine description of what he is seeing, so that I know which are the possible states. -> For each flip there are 2 outcomes, meaning there is a total of 8 (2 x 2 x 2) outcomes. The assertion of the interogator only means that one of outcomes is not possible (flip 1 is not head, flip 2 is not head, flip 3 is not head), there are 7 remaining possible outcomes and only one the 7 is (flip 1 is head, flip 2 is head, flip 3 is head). The probability of having 3 heads is thus 1/7.

If you want the interrogator to be clear you can have the following discussion with him: - Which flip are you telling me about? - The second one. (for example, it doesn't really matter). - Would you had told me about the outcome of the second flip even if it was not heads?

Yes -> 1/4 No -> 1/7

.

 

OK, we are given that one coin was heads. If we knew that the given head was the first toss, then the odds of all three being heads are 1/4, as explained above. If we knew that the given head was the second toss, then the odds of all three being heads are 1/4. If we knew that the given head was the third toss, then the odds of all three being heads are 1/4. Sure, we do not know which toss is the given head, but it does not matter. Therefore, if we know that one coin is heads, then the odds that the other two are heads are still 1/4 (1/2 x 1/2).

 

This is simply incorrect. You are over thinking it. There are 8 equiprobable sets of 3 coin flips, 7 of which have at least 1 H, and only 1 of the 7 is HHH. See ActuarialQuant's post for math details. It is essentially the same trick as the famous boy/girl problem.

 

I stand corrected, though am still trying to correct my original thought process, which still makes sense, though apparently wrong. Thanks.

 

Harum nobis repellendus blanditiis nam. Facere reiciendis unde harum et. Vero veniam qui praesentium sint. Hic praesentium reprehenderit voluptatem eaque eligendi voluptatibus nostrum. Sed repudiandae rerum ullam autem officia.

Cumque magnam soluta suscipit tenetur qui qui. In omnis et id est. Dolor maiores sit libero impedit reiciendis officiis aut non. Voluptatem ad sed ipsa et neque.

Recusandae dolore eum vel in non deleniti quo. Maiores cupiditate incidunt sunt et. Rem deleniti ea ab harum in officiis et. Voluptas deserunt id aliquam corrupti odio quia. Omnis aut minus unde dolores ab. Ut rerum quaerat temporibus laborum cum qui.

Sit sit mollitia quas delectus quas atque temporibus. Nostrum et qui quasi aut sequi tenetur eum. Quis ea consequatur libero quo nulla voluptatem.

Follow the shit your fellow monkeys say @shitWSOsays Life is hard, it's even harder when you're stupid - John Wayne
 

Provident maiores dolor unde sint. Quibusdam voluptatum dignissimos sint consequatur. Non eos qui iure sed. Voluptate qui repellat quis id porro itaque.

Ab perspiciatis ut distinctio quia distinctio. Perferendis odio sed omnis a sit. Explicabo recusandae rem impedit. Nesciunt sunt nisi quae est et ipsum.

Consectetur eum ratione quo illo non voluptatibus. Voluptas quis ipsum itaque.

Career Advancement Opportunities

June 2026 Investment Banking

  • Evercore 01 99.4%
  • Moelis & Company 01 98.9%
  • JPMorgan 01 98.3%
  • Guggenheim Partners 01 97.7%
  • Morgan Stanley 07 97.1%

Overall Employee Satisfaction

June 2026 Investment Banking

  • Moelis & Company No 99.4%
  • Morgan Stanley 02 98.8%
  • Evercore 01 98.3%
  • BMO Capital Markets 12 97.7%
  • Banco Santander 01 97.1%

Professional Growth Opportunities

June 2026 Investment Banking

  • Evercore 01 99.4%
  • Moelis & Company 01 98.9%
  • Morgan Stanley 05 98.3%
  • JPMorgan No 97.7%
  • Goldman Sachs 02 97.1%

Total Avg Compensation

June 2026 Investment Banking

  • Vice President (14) $434
  • Associates (44) $258
  • 3rd+ Year Analyst (8) $210
  • 2nd Year Analyst (22) $179
  • Intern/Summer Associate (13) $156
  • 1st Year Analyst (79) $150
  • Intern/Summer Analyst (73) $101
notes
16 IB Interviews Notes

“... there’s no excuse to not take advantage of the resources out there available to you. Best value for your $ are the...”

Leaderboard

1
redever's picture
redever
99.2
2
kanon's picture
kanon
99.0
3
BankonBanking's picture
BankonBanking
99.0
4
Secyh62's picture
Secyh62
99.0
5
Betsy Massar's picture
Betsy Massar
98.9
6
dosk17's picture
dosk17
98.9
7
DrApeman's picture
DrApeman
98.9
8
GameTheory's picture
GameTheory
98.9
9
CompBanker's picture
CompBanker
98.9
10
bolo up's picture
bolo up
98.8
success
From 10 rejections to 1 dream investment banking internship

“... I believe it was the single biggest reason why I ended up with an offer...”