Probability (1 Viewer)

[Trev]

Come on! We do this in General maths!

Well, it seems in general maths you don't learn to cancel like terms

 

[...]

The trick is that you assume that the first probability is 1 over 365.

*zing*

you just assumed the answer

 

[NT-social]

people, correct me if i'm wrong but doesn't the questiion say on the same day...(here i'm trying to make life much easier)...meaning u know, monday , tuesday... so it's only concerning got to do with 7

 

[NT-social]

1/7 possibly...lol...

 

[Stefano]

This question is a LOT harder than it seems.

I'm still unsure as to whether the answer is:
1. 1/365
2. 1/(365)^2
3. 1/7
4. 1/49

:S

 

[Jago]

it's 1/365

i'm pretty sure they mean on the same date. And since the question isn't "what is the probability that 2 people have same birthday on _________ " it won't be b nor d.

 

[serge]

I'm probably going to sound incredibly stupid for this but...wouldn't it just be (1/365)²
You *could* also do it via Binomial Distrubition...but what would be the point?

i think kami answered the question of picking one day in the year and then
figuring out the probability that two people have their birthday on that day

that's why if you increase the n, the probability will become smaller, as stefano said

 

[haboozin]

This question is a LOT harder than it seems.

I'm still unsure as to whether the answer is:
1. 1/365
2. 1/(365)^2
3. 1/7
4. 1/49

:S

hey, why are people still having a cry over this.

I am positive the answer is 1/365.

and is given by the formula 1 - 365pn/365^n (for any values of n)

it was even in one of the questions in cambridge i think.
Also it was even on tv ...channel 10 .. tuesday 8:30 .. that brainazoid thing which said that 50% chance if there are 23 people.. and when you put 23 as n you get 50%.

whoever sadi 1/7 or 1/49 has some problems..sorry...but why is this so hard?

 

[KFunk]

Noone needs a formula, the general maths people have it right.

 

[kami]

ok, this is how I see it:
1. If someone's birthday can be any day in a year, what is the probability one other person will have a birthday on that day?
Person 1's birthday = 365 x (1/365) = 1
Person 2's birthday = 1/365 as it is fixed to the day that the first person's birthday is.
:. resultant probability = 1/365 x 1 = 1/365
2. If someone's birthday is one a given day in the year, what is the probability someone will share that persons birthday?
Person 1's birthday = 1/365 as it can only be on one day in that year.
Person 2's birthday = 1/365 for same reason
:. resultant probability = 1/365 x 1/365 = (1/365)²
3. If someone's birthday is on any day of the week, what is the probability that a second person will have a birthday on that day?
Resultant probability = 1/7 (using method applied in 1. but switch 365 for 7)
4. If someone's birthday is on a given day of the week, then what is the probability of them sharing a birthday?
Resultant probability = 1/49 (using method from 2. but switch 365 for 7)

 

[KFunk]

ok, this is how I see it:

2. If someone's birthday is one a given day in the year, what is the probability someone will share that persons birthday?
Person 1's birthday = 1/365 as it can only be on one day in that year.
Person 2's birthday = 1/365 for same reason
:. resultant probability = 1/365 x 1/365 = (1/365)²

I think the answer for that case is still 1/365. It seems to be the same question. The assumption you make in the original question is that "someone's birthday is on a given day in the year". The probability that another person of an unknown birthdate is also born then is 1/365.

You would get the answer (1/365)² if the question was: "Two people are in a room. What is the probability that they were both born on April the 21st?"

(*edit: If someone is born on a given day then the probability that they were born on that day is '1')

 

[kami]

I think the answer for that case is still 1/365. It seems to be the same question. The assumption you make in the original question is that "someone's birthday is on a given day in the year". The probability that another person of an unknown birthdate is also born then is 1/365.

You would get the answer (1/365)² if the question was: "Two people are in a room. What is the probability that they were both born on April the 21st?"

(*edit: If someone is born on a given day then the probability that they were born on that day is '1')

But if you ask someone for instance "is your birthday on 21st April" there is a 1/365 chance that it'll be a yes, ask a second person and the probability of them both having 21st April would be (1/365)² wouldn't it?
EDIT:I get it, I meant a condition like in what I just posted, not an assumption.