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# Edexcel S3 - Wednesday 25th May AM 2016 watch

1. If you were asked find the probability that a student Spends less than 1/3 more than a teacher on reading: P(S< 4/3 T) how you'd set it up? Where s is student and t is teacher
2. (Original post by Rkai01)
If you were asked find the probability that a student Spends less than 1/3 more than a teacher on reading: P(S< 4/3 T) how you'd set it up? Where s is student and t is teacher
Hard to understand phrasing of question the way you've put it but if it's P(S<4/3 T) then you write it as P(S-4/3T < 0) and then write A = S-4/3T. Following this you calculate the combined mean (Mean of S - 4/3 mean of T) and variance (variance of S + (16/9 x variance of T)) and solve it.
3. (Original post by coolguy123456)
Hard to understand phrasing of question the way you've put it but if it's P(S<4/3 T) then you write it as P(S-4/3T < 0) and then write A = S-4/3T. Following this you calculate the combined mean (Mean of S - 4/3 mean of T) and variance (variance of S + (16/9 x variance of T)) and solve it.

I just meant the initial set up, that is the hardest bit. The wording can be confusing at times that's all.
4. Question 7d, can someone explain, in quite some detail, would be great!

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf
5. (Original post by etmarks)
Question 7d, can someone explain, in quite some detail, would be great!

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf
Hope this helps
6. Please explain when we use the squared of the Variance in the linear combinations of random variables, when we combine distributions or when we do it normally ..
7. (Original post by economicss)
Hope this helps
brilliant thanks, just wondering how the distribution of (x̄-μ) was conjured?
8. (Original post by etmarks)
brilliant thanks, just wondering how the distribution of (x̄-μ) was conjured?
So because the distribution of x bar is in general (mu, sigma squared/n) then the value for (x bar-mu) will be (mu-mu) giving the 0 and we are told that the standard deviation is 3 grams so the variance is (9/n) then we substitute this in to the formula
9. Can someone for q5 explain why the variance is not the smae for the second part of the question:

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf

Why is it 106 and not 52!

Anyone have any resources on this, every time I get this wrong. Done chapter 1 and the review qs
10. (Original post by L'Evil Wolf)
Can someone for q5 explain why the variance is not the smae for the second part of the question:

https://869d950bf149eefd5cd2652b4001...%20Edexcel.pdf

Why is it 106 and not 52!

Anyone have any resources on this, every time I get this wrong. Done chapter 1 and the review qs
Hope this helps, let me know if you need more explanation
11. (Original post by economicss)
Hope this helps, let me know if you need more explanation
Thanks I got the same thing as in working out only the wrong variance, can I ask how you know when to square the variance please?! Like you have done 3^2*3^2 whereas I did 3*3^2 :/
12. (Original post by L'Evil Wolf)
Thanks I got the same thing as in working out only the wrong variance, can I ask how you know when to square the variance please?! Like you have done 3^2*3^2 whereas I did 3*3^2 :/
Does this make sense?
13. (Original post by economicss)
Does this make sense?
Thank you this is very helpful.

It is not explained in the edexcel book haha.
14. (Original post by L'Evil Wolf)
Thank you this is very helpful.

It is not explained in the edexcel book haha.
No probs

Zacken

Has madas made a mistake on question 2, by neglecting to do the modulus?

the answer should be double that I think?
16. (Original post by L'Evil Wolf)

Zacken

Has madas made a mistake on question 2, by neglecting to do the modulus?

the answer should be double that I think?
No he hasn't your finding it given it is greater by 15. Not the difference is 15.
Probability that X2 exceeds X1 by atleast 15 so X2-X1>=15

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17. (Original post by physicsmaths)
No he hasn't your finding it given it is greater by 15. Not the difference is 15.
Probability that X2 exceeds X1 by atleast 15 so X2-X1>=15

Posted from TSR Mobile
Explained very well thank you.
18. Hi guys
Anyone got any ideas for 6d? Looking at the mark scheme Im not sure I understand. Its from June 2015
19. (Original post by econam)
Hi guys
Anyone got any ideas for 6d? Looking at the mark scheme Im not sure I understand. Its from June 2015
Write out U bar in terms of other variables then rearrange
20. (Original post by 13 1 20 8 42)
Write out U bar in terms of other variables then rearrange
Could the conditional probabilitlity/ percentage points/ other S1 stuff be asked in S3.

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