Cie as level statistics question , please help

Find the probability that, in a random sample of 9 phone calls made by Moses, more than 7 take
a time which is within 1 standard deviation of the mean. [5]

Please do explain the steps as I don’t get it

You expect us to help you when you don’t show us the full question..?

When Moses makes a phone call, the amount of time that the call takes has a normal distribution with mean 6.5 minutes and standard deviation 1.76 minutes.
(i) 90% of Moses’s phone calls take longer than t minutes. Find the value of t. [3]
(ii) Find the probability that, in a random sample of 9 phone calls made by Moses, more than 7 take
a time which is within 1 standard deviation of the mean. [5]

I don’t think u need the whole question to solve it

To begin, what is the percentage of the population that falls within 1 s.d. of the mean? This should be clear from the diagram I have posted in your other thread. I will refer to this value as $p$.

Hence this is the chance that a randomly picked call is within 1 s.d. from the mean. We may now model the problem for each of the 9 calls in a way that we are looking whether each call is within the required region or not, so treat them as a Binomial distribution with probability $p$, hence answer the question from this distr.

I don’t know the answer for ii) please do tell me thoroughly, I don’t even understand .



When Moses makes a phone call, the amount of time that the call takes has a normal distribution with mean 6.5 minutes and standard deviation 1.76 minutes.
(i) 90% of Moses’s phone calls take longer than t minutes. Find the value of t. [3]


(ii) Find the probability that, in a random sample of 9 phone calls made by Moses, more than 7 take
a time which is within 1 standard deviation of the mean. [5]

I did... please stop making threads on questions you have already asked..

https://www.thestudentroom.co.uk/showthread.php?t=5250430

I did... please stop making threads on questions you have already asked..

https://www.thestudentroom.co.uk/showthread.php?t=5250430

I made another thread because I obviously did not get your answer , and I didn’t get any answer from your answer , I just got the idea , I said I do NOT know how to solve the question , so if u know it , then why r u making it harder for me to understand , what is wrong with you ? If u knew an answer I would just answer it in a clear way using the numbers I am given not make up P and things like that

We don't solve questions for you on this forum. If you want the answer then look at the mark scheme.

The best way you can understand stuff like this is if you work through it yourself and ask for more guidance if required.

We don't solve questions for you on this forum. If you want the answer then look at the mark scheme.

The best way you can understand stuff like this is if you work through it yourself and ask for more guidance if required rather than make more threads. I have merged the two threads.

I have worked through that question 10 times , I looked at videos and everything available , I still do not get it

What part of it is confusing you?

Is it finding the probability of landing a call duration within 1 s.d.?
Is it applying the Binomial distribution?

I don't get what the value of p is

What part of it is confusing you?

Is it finding the probability of landing a call duration within 1 s.d.?
Is it applying the Binomial distribution?

I don't get what the value of p is

as i said, p is the percentage of values that fall within 1 s.d. Of the mean. You don’t have to do any work to find it because you can refer to the diagram i posted on the other thread to get it.

The diagram shows that ~34.1% of the values fall between the (mean) and (mean 1sd). However we also have ~34.1% of the values between (mean - 1sd) and (mean).
So together we get that ~68.2% of the values fall within 1sd from the mean, hence this is your p value.

Can you take it from there with the binomial distribution?

9c8(0.6826)^8(0.3174) (0.6826)^9

Thank you very much

Looks good!

I believe for the exam you need to remember the % values for 1sd from the mean, 2sd and 3 sd so you wouldnt need to work them out every time. You can use the diagram to help you remember them