Wednesday puzzle

[sasguru]

1965(!) How fookin old are you!!!!!!!!

The question was recycled in my stats course 10 years ago.
I think it's a classic because it shows probability isn't as intuitive as it seems.
Am impressed with MyUserName - he's come very close, although (2) is slightly off.

[d000hg]

Nope. It was actually a question on the PPE exam for Hilary College, Oxford, 1965.
Was looking at my old Stats papers.

What a fun-packed life you lead.

[sasguru]

What a fun-packed life you lead.

Well I'm at home getting paid fook loads to do diddly squat.
So compared to you, you're right.

[DimPrawn]

Well I'm at home getting paid fook loads to do diddly squat.
So compared to you, you're right.

Yeah, there's a big market out there for highly paid, work at home, read old school papers and post on the internet experts.

[sasguru]

Yeah, there's a big market out there for highly paid, work at home, read old school papers and post on the internet experts.

It's a shame you're so limited, otherwise you would know that it's true.
Tick tock tick tock, feeling my bank balance rise as I post this. Nice feeling

[MyUserName]

Am impressed with MyUserName

That is not something I here very often!!!

although (2) is slightly off.

Hmmm - let me look again

2 people out of 3 share a birthday:

I thought it was 3 x (1/365 + 1/365) = 6/365

2 out of 3 share a birthday would be 1/365 + 1/365 as the first person could either be the same as the second or third.

Oooh I think I get it.

The first person could be the same as the second or third - 2/365
The second could be the same as the third 1/365

Hence would it be 1/365 + 2/365 = 3/365?

[Robinho]

Probably wrong...

All 3) 1/365^2
2) (364+364)/365^2
None) 364*363/365^2

[MarillionFan]

That is not something I here very often!!!



Hmmm - let me look again

2 people out of 3 share a birthday:

I thought it was 3 x (1/365 + 1/365) = 6/365

2 out of 3 share a birthday would be 1/365 + 1/365 as the first person could either be the same as the second or third.

Oooh I think I get it.

The first person could be the same as the second or third - 2/365
The second could be the same as the third 1/365

Hence would it be 1/365 + 2/365 = 3/365?

What about leap years?

[eek]

Along the right lines .....
In fact no (3) is spot on. As is no (1). Apologies.
2 is quite hard but if you have 1 and 3 you should be able to do it

That's embarrassing (for you)

At 11:03 you thought the answer was wrong
At 11:10 you realised the answer was correct.

Show some consistency Muppet and actually read and think before trying to look clever.

[Bunk]

1 - (364/365 * 363/365) = Probability of at least 2 of them having same birthday

1 - (364/365 * 363/365) - (1/365 * 1/365) = Probability that exactly 2 have same birthday