Birthday Line
Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.
At a movie theater, the manager announces that they will give a free ticket to the first person in line whose birthday is the same as someone who has already bought a ticket. You have the option of getting in line at any time. Assuming that you don't know anyone else's birthday, that birthdays are distributed randomly throughout the year, etc., what position in line gives you the greatest chance of being the first duplicate birthday?
Answer
You should try to be the 20th person in line.
Suppose you are the Kth person in line. Then you win if and only if the K1 people ahead all have distinct birthdays AND your birthday matches one of theirs. Let
A = event that your birthday matches one of the K1 people ahead
B = event that those K1 people all have different birthdays
Then
Prob(you win) = Prob(B) * Prob(A  B)
(Prob(A  B) is the conditional probability of A given that B occurred.)
Now let P(K) be the probability that the Kth person in line wins, Q(K) the probability that the first K people all have distinct birthdays (which occurs exactly when none of them wins). Then
P(1) + P(2) + ... + P(K1) + P(K) = 1  Q(K)
P(1) + P(2) + ... + P(K1) = 1  Q(K1)
P(K) = Q(K1)  Q(K) < this is what we want to maximize.
Now if the first K1 all have distinct birthdays, then assuming uniform distribution of birthdays among D days of the year, the Kth person has K1 chances out of D to match, and DK+1 chances not to match (which would produce K distinct birthdays). So
Q(K) = Q(K1)*(DK+1)/D = Q(K1)  Q(K1)*(K1)/D
Q(K1)  Q(K) = Q(K1)*(K1)/D = Q(K)*(K1)/(DK+1)
Now we want to maximize P(K), which means we need the greatest K such that P(K)  P(K1) > 0. (Actually, as just given, this only guarantees a local maximum, but in fact if we investigate a bit farther we'll find that P(K) has only one maximum.) For convenience in calculation let's set K = I + 1. Then
Q(I1)  Q(I) = Q(I)*(I1)/(DI+1)
Q(I)  Q(I+1) = Q(I)*I/D
P(K)  P(K1) = P(I+1)  P(I)
= (Q(I)  Q(I+1))  (Q(K2)  Q(K1))
= Q(I)*(I/D  (I1)/(DI+1))
To find out where this is last positive (and next goes negative), solve
x/D  (x1)/(Dx+1) = 0
Multiply by D*(D+1x) both sides:
(D+1x)*x  D*(x1) = 0
Dx + x  x^2  Dx + D = 0
x^2  x  D = 0
x = (1 +/ sqrt(1  4*(D)))/2 ... take the positive square root
= 0.5 + sqrt(D + 0.25)
Setting D=365 (finally deciding how many days in a year!),
desired I = x = 0.5 + sqrt(365.25) = 19.612 (approx).
The last integer I for which the new probability is greater than the old is therefore I=19, and so K = I+1 = 20.
Computing your chances of actually winning is slightly harder, unless you do it numerically by computer. The recursions you need have already been given.
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Comments
Princess_Neopia
Dec 15, 2002
 You must have worked REALLY, REALLY hard. Your excellent at this. 
(user deleted)
Mar 25, 2003
 This is something!!! 
od1
May 02, 2003
 OH MY! 
beanie89
Aug 30, 2003
 uh....right. 
Varthen
Jul 02, 2004
 This teaser should be called...
HEADACHE IN A BOX!!!!!!!!!!!!!!!!!! 
trickster2005
Sep 10, 2005
 r u a genius? 
lessthanjake789
Jan 14, 2006
 i remember hearing about the birthday phenomenon but didnt quite understand it (i was sleeping in math class at the time) but it was quite interesting and i think this is a "practical" application of the numbers. nicely done and quite interesting. 
glenn222
Apr 13, 2006
 That was my next guess... 
NomadShadow
Apr 25, 2006
 I really loved it, I got the answer using difference equations.
Anyway, what if someone had a twin brother! wouldn't be better to stand second in line with the brother standing first;) 
Unstumpable
Jun 20, 2006
 wow, that would be annoying to type all of that, but the probibility works out all right, good teaser 
Dedrik
Aug 09, 2006
 Good problem 
foraneagle2
Oct 06, 2006
 Classic probability question. Another way of saying it: With as few as 20 random people in a room, the chance of two having the same birthday > 50%. 
ztodd
Oct 21, 2006
 That's not saying quite the same thing  that's just saying that the probability that either you or someone ahead of you will win is > 50%, but it doesn't necessarily mean you have the best chance. 
pearl5608
Oct 25, 2006
 Great teaser. i'm in AP Statistics in my high school, its my favorite class, this was an interesting problem 
roaddevil
Feb 08, 2007
 What if there is only two people at the movies? You can't figure probability with no numbers to start with! 
chickwithbrains
Sep 13, 2007
 uhhh.... what what what what??? Holy cow, I definitely didn't get this one!! !!!!! 
ZJT_05
Feb 26, 2008
 One Word  WOW. 
Anushka
Apr 11, 2008
 ver clever. and by the way...u must have a lot off free time on ur hands 
javaguru
Dec 08, 2008
 Great problem!
I'm surprised given how long this problem has been posted that nobody bothered to give the probability of winning the free ticket, which is approximately 3.0628%. 19th place in line is almost as good at 3.0608%. This is a pain to calculate recursively by hand but very easy using a spreadsheet.
At 20 people the probability that two people have the same birthday is approximately 41.1% and the cumulative probability that someone in line before you wins the ticket is approximately 36.67%.
Also, the point at which the probability of two people having the same birthday exceeds 50% is with 23 people, not 20. 
(user deleted)
May 12, 2009
 Unfortunately, the author's longwinded answer is completely WRONG.
The correct answer is approx. 23.3 Rounded to the nearest whole number, the correct answer is 23.
As the previous posting correctly stated, the point at which the probability of two people having the same birthday exceeds 50% is with 23 people, not 20, and not 19. 
javaguru
May 14, 2009
 Janeys
The answer given for the teaser is correct. You need to take into consideration the CUMULATIVE probability that someone in front of you wins the ticket before you get to the front.
The fact that at 23 people there is a better than 50% chance that two have the same birthday isn't really relevant to the problem. I just included that in my previous comment as a point of interest. 
rlc327
Jul 17, 2010
 i would think the last person in line at any time where position P < or = 20. 
normadize
Mar 29, 2012
 I know this is old but it took me only a few minutes to solve and my solution is quite different, so I thought I might as well post it. In my opinion the answer given is convoluted.
Let p(k) be the probability that the first duplicate bday is at person k+1 in the line, i.e. the first k people have all different bdays and the bday of person k+1 matches one of the previous k bdays. No need for any conditional probabilities (why the Bayes equation in the answer?) since all events are independent and p(k) can be expressed directly:
p(k) = 365/365 * 364/365 * ... * (365  k + 1)/365 * k/365
(easily verifiable thoruhg simulation for those who doubt it)
It says that the 1st person can have any bday (365/365), the 2nd person has a different bday than the 1st (364/365), and so on, and person k+1 has one of the first k bdays (k/365).
We just need to maximize p(k) as a function of k, that's it. It is easy to show it has only one maxima. The current form is ugly but we can do a trick, that is to find an equivalent, continuous function. We can write p(k) as:
p(k) = k/365^(k+1) * product_of k {from 365k+1 to 365}
Taking the natural log:
ln(p(k)) = ln(k/365^(k+1)) + sum ln(k) {from 365k+1 to 365}
The sum of discrete logs can be approximated by an integral of the continuous log function given that the terms in the sum are large (whoever knows of Sterling's factorial approx proof is already familiar) and so we can consider the continuous function q(x):
q(x) = ln(x/365^(x+1)) + integral ln(x) dx {from 365x+1 to 365}
using parts:
q(x) = ln(x/365^(x+1)) + 365 ln 365 + (366x) ln(366x)  x + 1
Differentiating, and equating to zero, we get:
1/x = ln (365 / (366x)) ... or ... exp(1/x) = 365/(366x)
This has a unique solution since exp(1/x) is decreasing on [1,365] while 1/(366x) is increasing on [1,365]. This is computed numerically to give x = 19.3676 so then our k+1 = 20, which is the solution. 
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