to ask if you can answer a question re probability (Maths question)

[Fainne]

So, say I have 20 cards in a pack.

I pick one. It's the Ace of Diamonds let's say for argument's sake.

I then pick another one out of the same pack of 20 cards the following day.

Am I correct in saying that the odds of me picking the same card is a multiple of the single odds?

So 1/20 x 1/20 = 1/400

?

Because I've someone telling me the odds are still 1/20 that I'll pull the same card.

obviously the rules of cumulative probability change when you are talking about tarot cards rather than a standard deck....

A lot of things change when you are talking about tarot cards rather than a standard deck. Besides, with 20 cards, it seems the OP isn't playing with a full deck anyway.

A lot of probability isn't all that intuitive - as already mentioned on the thread just have a look at the Monty Hall problem or the "how many people you need for it to be likely that 2 of them share a birthday".

A great book that explains a lot of this in an accessible way is "How not to be wrong".

What the hell? Poster posts vague thread, insults those who are more knowledgeable, then flounces when it's pointed out they're both wrong AND a dick...

average day on mumsnet I suppose!

Sure it's been said but I would love to offer the OP odds of 36/1 on me getting a double if I roll 2 dice.

Will agree with others that it is 1 / 20. You'will succedd if you either pick card 1 twice (1 / 400) OR card 2 twice (1 / 400) OR card 3 twice (1 / 400)... add twenty 1 / 400 chances together, and you get a 1 / 20 chance of a match.

I like a probability question (and I have an A level maths!) but this is one of three weirdest threads I've ever read, and that's saying something!

Day one could be ANY card. Odds of picking any card are 20/20 (i.e. 1) it will definitely happen. It doesn't matter which one you pick the first day.
Once you know what it is, odds of picking it the next day are 1/20.

The only way for it to be 1/400 would be if you named a specific card before picking on the first day.

Ok then.

What are the odds of me pulling the same card 5 days in a row. An unspecified card in the first place? Still 1/20?

@Fainne

I just reread the opening post.

On day one you picked a card at random.
This card is the choose card. You were never going to pick a wrong card as you don't mind which card you pick.

So your probability only kicks in on day two.
You have 20 cards to pick but only 1 card if the right card. 1/20...

I'm still picking at random, not wanting a particular card (not that it would matter what I wanted). But you're all saying that the odds of me pulling the same card 5 days in a row is the same?

20x(1/20)x(1/20)x(1/20)x(1/20)x(1/20) = 1/160 000

It's actually 25 'cards' I'm actually dealing with. I just picked 20 as a nice round number.

But you're all saying that the odds of me pulling the same card 5 days in a row is the same?

Who mentioned 5 days in a row?
You really still don't understand the probabilities, do you?

So you all agree that if I want the Ace of Diamonds, I get the Ace of Diamonds, then the odds of getting the Ace of Diamonds a second time is not 1/20.

It's actually 25 'cards' I'm actually dealing with.
And? Just replace 20 with 25.

It's not cards I'm dealing with here - it's something else. But I don't like the sly dig 'she's not dealing with a full deck'.
I'm actually not talking about cards at all!

So you all agree that if I want the Ace of Diamonds, I get the Ace of Diamonds, then the odds of getting the Ace of Diamonds a second time is not 1/20.

If you phrase it like that, it's 1/400.

If you phrase it as any card (unspecified) twice in a row, then it's 1/20.

I'm actually not talking about cards at all!
It doesn't matter.

If you don't like digs, then don't make them.

For an unspecified card it's 1/1 x 1/20 x 1/20 x 1/20 x 1/20 = 1/160000 because once you'vw set which card you're looking at, it's a 1 in 20 chance of picking it out.

Ok. You're all right and I'm all wrong.
Happy now?

The bottom line is that it's different depending on whether you want a specific card the first time.

So you all agree that if I want the Ace of Diamonds, I get the Ace of Diamonds, then the odds of getting the Ace of Diamonds a second time is not 1/20.

The probability of getting an ace of diamonds on any random pick is 1:20,

If you’re asking for the probability of getting the ace of diamonds twice in a row (replacing in between) is 1/400.

What you are missing is that, if you don't want a particular card on day one then the first card doesn't really count towards the probability. It is once you have drawn the first card that you start calculating as you have now set the parameters of the problem.

If you set the parameters before you draw the first card then it will count.

It really does depend on if you specify the card before or after you draw.

I want the Ace of Diamonds. Twice.