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S1 Help Please.

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"Given that it is 127 hours since Alice last charged her phone" means that it's been at least 127 hours since Alive charged her phone throughout the journey. So it could be anywhere between 127 hours and infinity hours since she last charged her phone.

Gotcha, thanks. :smile:

Reply 21

Glavien

For working out the variance of question 2(a) of this paper, I used the formula of the standard deviation squared to work it out, however the model answers uses Sxx/n. Why did they use that?

Model Paper: https://1a388b28f5ce9318f837a64962fc3ceab9c61da8.googledrive.com/host/0B1ZiqBksUHNYU2s1SVY1VDN2Q0U/January%202014%20(IAL)%20MA%20-%20S1%20Edexcel.pdf

Reply 22

Zacken

Original post by Glavien

Think about the definition of Sxx in terms of summations and how it compares to the formula of variance in terms of summations.

I'd use the normal formula tbh.

Reply 23

Glavien

Original post by Zacken

Think about the definition of Sxx in terms of summations and how it compares to the formula of variance in terms of summations.

I'd use the normal formula tbh.

Ahhh, yeah I understand it now, thanks. Whats the 'normal formula'?

Reply 24

Zacken

Original post by Glavien

Ahhh, yeah I understand it now, thanks. Whats the 'normal formula'?

The one that you used.

Reply 25

Glavien

Original post by Zacken

The one that you used.

Cool, thanks! :smile:

Reply 26

Zacken

Original post by Glavien

Cool, thanks! :smile:

No problem.

Reply 27

Glavien

What is the best way of representing the information for question 7 in order to make the question clearer, tree diagram or venn diagram? I don't understand how you can work out P(LH intersection F) for part (b).

Paper: https://1a388b28f5ce9318f837a64962fc3ceab9c61da8.googledrive.com/host/0B1ZiqBksUHNYU2s1SVY1VDN2Q0U/January%202014%20(IAL)%20QP%20-%20S1%20Edexcel.pdf

Reply 28

Zacken

Original post by Glavien

Tree-diagram.

Reply 29

Glavien

Can you explain question 7(f) of this question please? I'm really confused, I understand the P(S=1) = 0.2 but don't know what to do next.

Paper: https://1a388b28f5ce9318f837a64962fc3ceab9c61da8.googledrive.com/host/0B1ZiqBksUHNYU2s1SVY1VDN2Q0U/June%202013%20(R)%20QP%20-%20S1%20Edexcel.pdf

Reply 30

Zacken

Original post by Glavien

It's just saying "what is the probability that the score on the first die is 1 and the score on the second die multiplied by the score on the first die is less than 5).

So that's P(S1 = 1 and S2 = 1) + P(S1 = 1 and S2 =2) + P(S1 = 1 and S2 = 3) + P(S1 = 1 and S2 = 4)

since in all those cases, S1 = 1 and S1 * S2 < 5.

Then remember that P(S1 = 1 and S2 = 1) = P(S1=1) * P(S2 = 1) and the same for the others.

Reply 31

Glavien

Original post by Zacken

Thanks, what do the curly braces mean when stating S1=1?

Also, could the question be written as:

If S1 = 1 show that P(X<5) = 0.16

Reply 32

Glavien

For question 3(e) of this paper, would you work out the number of plants with a yield above (mean + standard deviation)? How would you do that as it the frequency somewhere in between the interval 15<=y<35 plus 4?

Paper: https://1a388b28f5ce9318f837a64962fc3ceab9c61da8.googledrive.com/host/0B1ZiqBksUHNYU2s1SVY1VDN2Q0U/June%202013%20(R)%20QP%20-%20S1%20Edexcel.pdf

Reply 33

Zacken

Original post by Glavien

Thanks, what do the curly braces mean when stating S1=1?

Just some technical stuff about events and sets, doesn't really mean anything in particular.

Also, could the question be written as:

If S1 = 1 show that P(X<5) = 0.16

I think so.

Original post by Glavien

Have a look at this.

Reply 34

Glavien

Original post by Zacken

Just some technical stuff about events and sets, doesn't really mean anything in particular.

I think so.

Have a look at this.

Thanks!

Reply 35

Glavien

For question 2(e)(iii), I would've thought when using the standard deviation formula the term sum(x^2f) would increase, as the value 7.88km is in the interval 6-10 so the midpoint used would be 8 and the frequency used is 11. Also the sum(f) will increase to 51. Thus, the standard deviation increasing. Can you please explain why this is not the case? Thanks!

Paper: https://1a388b28f5ce9318f837a64962fc3ceab9c61da8.googledrive.com/host/0B1ZiqBksUHNYU2s1SVY1VDN2Q0U/June%202014%20(IAL)%20QP%20-%20S1%20Edexcel.pdf

Reply 36

Zacken

Original post by Glavien

You're overcomplicating this. She travels an exact distance of the mean. This means that she contributes nothing to the standard deviation or variance. Remember that variance is sum of (x - mean)^2 or something, in her case x = mean so her part of the sum is mean - mean = 0.