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Stats Binomial and perms combs question watch

1. Hi, So I've been working through a past paper, but got stuck on question 3 part 2 and question 9 part 1 b on the following paper. http://www.ocr.org.uk/Images/79328-q...atistics-1.pdf

For question 3 i worked out the probability of X=2 which is 0.27527...

Then I though since we are given two values to find the probability of only 1 of them we divide by 2, but it's wrong.

For question 9, I just don't know how to do it, applications of perms and combs always confuse me.
2. (Original post by Music99)
Hi, So I've been working through a past paper, but got stuck on question 3 part 2 and question 9 part 1 b on the following paper. http://www.ocr.org.uk/Images/79328-q...atistics-1.pdf

For question 3 i worked out the probability of X=2 which is 0.27527...

Then I though since we are given two values to find the probability of only 1 of them we divide by 2, but it's wrong.

For question 9, I just don't know how to do it, applications of perms and combs always confuse me.
For q 3 part 2 find P(X=2) then set up a second distribution - with Y~B(2,P(X=2))
3. Bump.
4. (Original post by Music99)
Bump.
For 3 (i),

For 9,

use

"n choose r"

and note that odd numbers must end with an odd digit.

The last part involves simple probabilities.
5. (Original post by Indeterminate)
For 3 (i),

For 9,

use

"n choose r"

and note that odd numbers must end with an odd digit.

The last part involves simple probabilities.
It's the second part of 3. I tried the method above and i didn't get the correct answer. I'm retrying question 9 now.
6. (Original post by Music99)
It's the second part of 3. I tried the method above and i didn't get the correct answer. I'm retrying question 9 now.
Given

let

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