Inspired by Vaibhav Ojha

In a lottery draw of 100 tickets, there is 1 winner and 99 losers. I bought 2 tickets, looked at both of them and announced (tautologically) that one of them is a loser (you are not told which ticket is the loser). What is the chance that both of the tickets are losers?

49 50 \frac{49}{50} 98 99 \frac{98}{99} 1 2 \frac{1}{2} 99 100 \frac{99}{100}

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

3 solutions

Since there is only one winner, If you buy two tickets it is guaranteed that at least one of the tickets is a loser. Thus stating that one of your tickets is a loser is a tautology, and hence provides no "useful" information regarding what the "other" ticket might be. Thus the desired probability is just the probability that both tickets are in fact losers, which is the product of the probabilities of choosing one loser followed by another, (without replacement), i.e.,

99 100 98 99 = 98 100 = 49 50 . \dfrac{99}{100} * \dfrac{98}{99} = \dfrac{98}{100} = \boxed{\dfrac{49}{50}}.

How can one approach this problem without knowing that a tautology is involved? IE What would be the standard approach for this type of problems?

Calvin Lin Staff - 5 years, 11 months ago

Log in to reply

P(A|B) = \frac{P(A)}{P(B)}
P(A) = \frac{99}{100} \times \frac{98}{99}
P(B) = 1
in other words the additional information provided is by no means useful !

Jacob Antony - 5 years, 10 months ago

Log in to reply

That's great!

Calvin Lin Staff - 5 years, 10 months ago

The problem is worded badly. This seems to be the case with a lot of probability problems. Precision in the way the way these kinds of question are worded is absolutely critical.

The question says that "you are not told which ticket is the loser" but then asks "what is the probability that the OTHER ticket is also a loser."

If you don't know which ticket is the loser, it is not clear what is meant by the OTHER ticket. The phrase "other ticket" only has meaning to the one who knows which ticket was initially declared a loser. We are not told whether that person looked at only one ticket and has not yet looked at the other ticket (in which case the probability is 98/99 that it is also a loser) or looked at both and is stating that at least one ticket is a loser. A clearer way to state this problem would be "You are told that at least one of the two tickets is a loser. What is the probability that they are both losers?" Given the use of the phrase "other ticket" I went with the first interpretation.

An interesting twist on this question is:

You look at one of two tickets and see it is a loser. You don't know whether the second ticket is a winner or loser. You then shuffle and choose one of the two tickets at random. What is the probability it is a loser?

Sean McCloskey - 4 years, 3 months ago

Log in to reply

Indeed. There are several possibilities: a) the person didn't look at either ticket and just deduced at least one ticket had to be a loser b) the person only looked at one ticket and reported that it was a loser.
c) the person looked at both tickets and reported the result of the first ticket (actually this is the same as b) d) the person looked at both tickets and reported that at least one was a loser (actually this is the same as a)

I interpreted the statement to be b), because the other interpretation doesn't make any sense. Who buys two lottery tickets and says "at least one is a loser." That's a tautological and uninteresting statement.

If this question wants to be framed along the lines of the classic Monty Hall question, it needs to be worded more carefully. If all we're being asked is "What are the odds of buying two losing tickets?" it should be phrased as such.

Richard Desper - 4 years, 1 month ago

Log in to reply

I have added in "looked at both of them". I see how the ambiguity of knowing that event can lead to a different answer.

Calvin Lin Staff - 4 years, 1 month ago

Sean McCloskey clearly identifies the bad wording of the problem. I, too, considered 98/99 to be the correct answer.

John Link - 3 years, 6 months ago

I agree that the question is poorly worded and admits another logical interpretation. I took the interpretation that there are 99 "other" tickets, 1 of which is a winner. This leads to answer 98/99.

If you told me I have at least one loser in my pair, this can lead to the given answer.

I see the distinction, but find it subtle and the given assumptions open to interpretation.

Will Heierman - 4 years, 1 month ago

Log in to reply

Ah, I see where you're coming from.

I've rephrased the question to "both of the tickets are losers".

Calvin Lin Staff - 4 years, 1 month ago

I was trying to approach this problem using Bayes theorem, and I was getting a different answer. I would appreciate if someone could point out the flaw in this method.

Event A: When Ticket 1 is loser Event B: When Ticket 2 is loser

I am calculating the probability of Event A given Event B has already occurred. This, by Bayes Theorem reduces to-

P(A|B)=P(A&B)/P(B)

=(99C2/100C2) / (99C1/100C1) =((99* 98/100* 99)/ 99*100) =98/99

@Jacob Antony I could not follow your logic when you took P(B)=1.

Rahul Soni - 5 years, 2 months ago

Log in to reply

I think the issue is that we are not told which ticket is the loser, just that one of then is, so your choice of events A and B are not quite in line with the question at hand. In this case, you'll need to let A be the event that both tickets are losers and B be the event that (at least) one of the tickets is a loser. Then since there is only one winning ticket, any choice of two tickets will include at least one losing ticket, implying that P(B) = 1. This in turn implies that P(A|B) = P(A&B), which is, as you have noted, (99C2)/(100C2) = 98/100 = 49/50, in agreement with the posted solution.

Brian Charlesworth - 5 years, 2 months ago

Log in to reply

So the key is we don't know which ticket is announced as loser. If the question mentioned that "you are told which ticket is the loser", then the answer become 98/99.

Lee Hau - 4 years, 6 months ago

Log in to reply

@Lee Hau Indeed. If the question was "The ticket in my right hand is a loser, what is the probability that the ticket in my left hand is a loser", then the answer is 98/99.

Calvin Lin Staff - 4 years, 6 months ago

The events to consider are A: both tickets are losers and B: at least one ticket of the two is a loser. P(B) is clearly 1 and P(A&B) = P(A) = 98/99 * 99/100 = 98/100 = 49/50. Thus P(A|B) = (49/50)/1 = 49/50

Richard Farrer - 2 years, 6 months ago

" If you buy two tickets it is guaranteed that at least one of the tickets is a loser. " But we are not told whether the person looked at both tickets or not before making the announcement.

Richard Desper - 4 years, 1 month ago

I got it wrong (choosing 98/99).
But I now see this as a variant of the "Let's Make A Deal" choose one door out of three, then Monte shows you one of the doors you didn't pick. The correct solution is to then choose the other door Monte offers you (p=2/3 success).
It is indeed true that there is a 2/100 = 1/50 chance that the winning ticket is in the two tickets chosen. Being told one is a loser is worthless information. It still remains that there is a 1/50 shot the other ticket is a winner.

Stephen Rasey - 2 years, 9 months ago
Nicolas Vilches
Jul 18, 2015

We can do this using laplace law:

Favorable cases: ( 99 2 ) \binom{99}{2}
Posible cases: ( 100 2 ) \binom{100}{2}

So, our probability is:

( 99 2 ) ( 100 2 ) = 49 50 \frac{\binom{99}{2}}{\binom{100}{2}}=\frac{49}{50}

Elegant and unbelievable. Can you prove when you can apply this method?

Lu Chee Ket - 4 years, 4 months ago

I meant what Laplace Law said.

Lu Chee Ket - 4 years, 4 months ago
Lu Chee Ket
Feb 1, 2017

First step: ( 1 100 W (\frac{1}{100} W , 99 100 L ) \frac{99}{100} L) of total 1 1 \frac{1}{1} alone;

Second step: ( 0 99 W (\frac{0}{99} W , 99 99 L ) \frac {99}{99} L) + ( 1 99 W (\frac{1}{99} W , 98 99 L ) \frac{98}{99} L) of total 2 2 \frac{2}{2} alone.

Hence 99 100 L × 98 99 L = 98 100 L ² = 49 50 L ² \frac{99}{100} L \times \frac{98}{99} L = \frac{98}{100} L² = \frac{49}{50} L²

Verified that 100 N 100 \frac{100 - N}{100} shall be the chance of having all losers, particularly when N = 99 and N = 100, checked that all branches add up to 1, we can be confident that the answer is 49 50 \frac{49}{50} particularly when all other options given are not likely to be correct.

Answer: 49 50 \boxed{\frac{49}{50}}

It is obvious that if you buy 2 tickets 1 is gonna be a loser since there's only one winner, and now if we buy 2 tickets out ou 100, the probability to win is 49/50

Joan Arnalot - 4 years, 4 months ago

So, if i have two tickets and one of them is a loser then the other is a winner. So the chances of having two losing tickets is zero. Not an option listed. Implication is that the statement should be "at least one is a loser" but then, as there is only one winner in this lottery, such a statement is unnecessary.

Matthew Civil - 3 years, 8 months ago

The emailed problem question asks "What is the chance that the other ticket is also a loser?" which corresponds to answer 98/99. Then the question is different on this page "What is the chance that both of the tickets are losers?" which is 98/100 = 49/50. It is irritating to have the emailed question different from that on the web page.

A Former Brilliant Member - 2 years, 8 months ago

Log in to reply

That's still the case, and still irritating, six weeks later.

David Entwistle - 2 years, 7 months ago

1 pending report

Vote up reports you agree with

×

Problem Loading...

Note Loading...

Set Loading...