# Probability no male- female pairs share same birthday

There are 8 people in a room. There are 4 males(M) and 4 females(F). What is the probability that there are no M-F pairs that have the same birthday ? It is OK for males to share a birthday and for females to share a birthday. Assume there are $$10$$ total birthdays.

I give a solution below. Not sure if is correct and is there a more general way to approach it ? I break it into 5 cases-summing these cases gives the total ways M-F do not share. If divide the sum by $$10^8$$ would obtain desired probability.

Case 1: all men have different birthdays $$N_1 = 10 \cdot 9 \cdot 8 \cdot 7 \cdot (10-4)^4$$

Case 2: one pair men exact + two single men $$N_2 = {\sideset{_{10}}{_1} C} \cdot {\sideset{_4}{_2} C} \cdot 9 \cdot 8 \cdot (10-3)^4$$

• the first term chooses the single BD for the pair of men.
• The second term selects the 2 men in the pair.
• The $$9\cdot 8$$ are the number of ways the two single men can choose their birthdays.
• The final term is the number of ways the $$4$$ woman can select the remaining $$10-3 = 7$$ birthdays which do not equal the men which have used $$3$$ birthdays.

Case 3: two pair men exact $$N_3 = {\sideset{_{10}}{_2} C} \cdot {\sideset{_4}{_2} C} \cdot {\sideset{_2}{_2} C} \cdot (10-2)^4$$

Case 4: one triple and one single man $$N_4 = {\sideset{_{10}}{_1} C} \cdot {\sideset{_4}{_3} C} \cdot {\sideset{_1}{_1} C} \cdot {\sideset{_9}{_1} C} \cdot (10-2)^4$$

Case 5: all men have same birthday $$N_5 = {\sideset{_{10}}{_1} C} \cdot (10-1)^4$$

The sum of Case $$1$$ to $$5$$ is the total ways for no M-F pairs. The last term in each case is the number of permutations of the 4 woman with $$(10-k)^4$$ choices where $$k$$ is the number of unique birthdays used up for the men. I do not believe the order of the people matters: I calculate assuming all the men come first. Please comment on my approach.

• Can you edit your question to make it consistent? – scoopfaze Jan 26 '20 at 0:36
• @ironX: I think what's meant is to assume that there are $10$ possible days of the year instead of $365$. – joriki Jan 26 '20 at 0:41
• Yes instead of 365 days in a year assume it is a special planet which only has 10 days. – user263904 Jan 26 '20 at 0:53
• @scoopfaze : what do you mean not consistent? – user263904 Jan 26 '20 at 0:55
• Look at @ironX's comment – scoopfaze Jan 26 '20 at 0:56

Your calculation has a minor error in that case 4 should end with *(10-3)^4 rather than *(10-2)^4.

If you correct that and add the numbers up then you would get $$19550250$$. Dividing by $$10^8$$ would then give the probability of $$0.1955025$$

Generalising this is a little messy because your cases 2 and 3 each count possibilities for the mean having two birthdays between them. There is a way round this by using Stirling numbers of the second kind and you could say something like

If there are $$d$$ days in a year, and $$m$$ men and $$w$$ women with their birthdays independently and uniformly distributed across these days, then the probability that there are no cases of a man and a woman sharing a birthday is $$\frac{d! }{d^m}\sum\limits_{n=1}^{\min(m,d)} \frac{S_2(m,n) }{(d-n)!}\left(1-\frac{n}{d}\right)^w$$
where $$S_2(x,y)$$ is the corresponding Stirling number of the second kind.

If you applied this to your example with $$d=10, m=4,n=4$$, it would give $$362.88\left(\frac{1\times 0.9^{4}}{362880} + \frac{7\times 0.8^{4}}{40320} + \frac{6\times 0.7^{4}}{5040} + \frac{1\times 0.6^{4}}{720}\right)=0.1955025$$

Let $$A$$ be the event no M-F pair share the same birthday. Let $$B_1$$ be the event all females share ONE birthday.

Let $$N(A \cap B_1)$$ be the number of possible configurations realizing the event $$A \cap B_1$$.

I think $$N(A \cap B_1) = {10 \choose 1} {9 \choose 1} + \left [ {10 \choose 2}* {4 \choose 2} \right ] * {8 \choose 1} + \left [ {10 \choose 3} * 3! * 3 \right ] *{7 \choose 1} + \left [ {10 \choose 4} * 4! \right ] * {6 \choose 1}$$

The first term is {all men share the same birthday} $$\cap B_1$$

the second term is {all men share two distinct birthdays } $$\cap B_1$$

the third term is {all men share three distinct birthdays } $$\cap B_1$$

the fourth term is {all men share 4 distinct birthdays} $$\cap B_1$$.

I think we can calculate $$N(A \cap B_i)$$ for $$i = 2,3,4$$ and then the result would be:

$$\frac{N(A \cap B_1) + N(A \cap B_2) + N(A \cap B_3) + N(A \cap B_4)}{10^8}$$

Let me know any errors.

• thanks for attempting this problem but I have decided this question is not clear enough so I am going to delete it . Sorry. – user263904 Jan 27 '20 at 15:27