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# Edexcel S2 - 27th June 2016 AM

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1. (Original post by Music With Rocks)
Attachment 556905
For part (b) why is the mode not 1?

Surely ?
(Original post by Music With Rocks)
I am sorry but I don't understand what you are saying I am very confused haha
I'm not quite sure either you've asked about f(1) but then calculated f(2).
2. (Original post by fpmaniac)
June 2014 (R) question 3d?

https://57a324a1a586c5508d2813730734...%20Edexcel.pdf

I cant get my head around the differences wording in 3c and 3d.
The difference is that in 3c you are finding a FIXED probability so you can model 3d binomially. You need to define a new variable.. I'll walk you through:

Ok, you have your answer to 3c don't you. You need to model this binomially with a different variable e.g. Y which would be the number of months with atleast 1 accident. So you now have Y~B(6.0.777) because you are looking at 6 months. You then look for the probability you want which in this case is EXACTLY 4 months. P(X=4) use the formula in the booklet and find the probability. Does this help ?
3. (Original post by economicss)
Please could you post the answer for part c, I keep getting a decimal for the mean and not a fraction, thanks
I don't have the answers I'm afriad they may well not be fractions, it's the method that's most important anyway I'll have a look at the answers in a bit and quote you again.
4. (Original post by SeanFM)
I dunno, I don't think it was particularly obvious. But you've seen it once now and that would be enough, and if you get asked something slightly different you can just kind of step back and think about how to solve it.
5. (Original post by SeanFM)
I'm not quite sure either you've asked about f(1) but then calculated f(2).
But surely the mode is the highest point of the pdf graph which is at f(2)?
6. For the June 2015 Paper:
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf

Question 3 (d): When they usually ask for the quartiles and the median(For other questions) , which function do you equate to 0.75 if there are more than one? Do you add up the F(x) of both then equate, or equate any one of them?
7. (Original post by Music With Rocks)
But surely the mode is the highest point of the pdf graph which is at f(2)?
Correct - that corresponds to a value of x =2. It's the input that matters, not the output.
8. http://qualifications.pearson.com/co...e_20150622.pdf
Q5d, someone help me please and that'll be the last you'll see of me until I begin screaming for help with C3, C4, FP2, FP3, M2, M3 and maybe D1.
9. (Original post by JammyW15)
For the June 2015 Paper:
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf

Question 3 (d): When they usually ask for the quartiles and the median(For other questions) , which function do you equate to 0.75 if there are more than one? Do you add up the F(x) of both then equate, or equate any one of them?
You have to work it out (if there's just two, take F(the upper limit of this function)) and see if it's more or less than 0.75. If it's more than 0.75, it's somewhere in the first function. If it's less, then it's definitely in the second function.
10. (Original post by SeanFM)
Correct - that corresponds to a value of x =2. It's the input that matters, not the output.
ohhhh... I get it now, thank you very much
11. (Original post by JammyW15)
For the June 2015 Paper:
https://57a324a1a586c5508d2813730734...%20Edexcel.pdf

Question 3 (d): When they usually ask for the quartiles and the median(For other questions) , which function do you equate to 0.75 if there are more than one? Do you add up the F(x) of both then equate, or equate any one of them?
Use the part of the CDF of which the upper quartile will lie in, which in this case is the second equation. Picture the graph in your head/sketch if struggling
12. (Original post by wr123)
The difference is that in 3c you are finding a FIXED probability so you can model 3d binomially. You need to define a new variable.. I'll walk you through:

Ok, you have your answer to 3c don't you. You need to model this binomially with a different variable e.g. Y which would be the number of months with atleast 1 accident. So you now have Y~B(6.0.777) because you are looking at 6 months. You then look for the probability you want which in this case is EXACTLY 4 months. P(X=4) use the formula in the booklet and find the probability. Does this help ?
If you do poisson for any question, the probability you find from that, is that always a fixed probability.
13. (Original post by AlphaArgonian)
http://qualifications.pearson.com/co...e_20150622.pdf
Q5d, someone help me please and that'll be the last you'll see of me until I begin screaming for help with C3, C4, FP2, FP3, M2, M3 and maybe D1.
binomial (4,p) where you should be able to get p quite easy
14. (Original post by SeanFM)
You have to work it out (if there's just two, take F(the upper limit of this function)) and see if it's more or less than 0.75. If it's more than 0.75, it's somewhere in the first function. If it's less, then it's definitely in the second function.
Ah Alright thank you!
15. (Original post by AlphaArgonian)
Use the part of the CDF of which the upper quartile will lie in, which in this case is the second equation. Picture the graph in your head/sketch if struggling
Oh okay thanks!
16. (Original post by 13 1 20 8 42)
binomial (4,p) where you should be able to get p quite easy
17. (Original post by JammyW15)
Ah Alright thank you!
Just in case there's 3 functions.. in general, as you'd expect, there's only 1 x value, or one of the functions, such that F(X) = 0.75, so equating to any one of them won't work - you first have to work out which one it actually is, usually by trial and error (and testing the ranges of each function, to use C3 lingo).
18. Do we need to know the proofs for mean and variance: (a+b)/2 and 1/12(b-a)^12?? If so, how do u derive them?
19. (Original post by SeanFM)
Sorry - I didn't get that notification (I'd show you proof of that if you want, but feeling too lazy ) - notifications are bugged at the moment if you've noticed.

You haven't integrated 3x/20 correctly, I think.
Oh dear rookie error... thank you!

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20. (Original post by AlphaArgonian)
use poisson

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