# StatsWatch

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Thread starter 3 weeks ago
#1
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]
z score is 1.282 but how they know whether its - or + ?
0
3 weeks ago
#2
(Original post by Sasha Shasha)
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]
z score is 1.282 but how they know whether its - or + ?
Just @sketch it. If 90% are greater than t, t must be less than the mean so the z score is ...
1
3 weeks ago
#3
(Original post by Sasha Shasha)
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]
z score is 1.282 but how they know whether its - or + ?
The probability in question is . The diagram for this is below.

The transformation into the z variable is given by . Hence, the small z value would be . Since is less than 6.5, will the difference t-6.5 be +ve or -ve??

1
Thread starter 3 weeks ago
#4
Thank You
0
Thread starter 3 weeks ago
#5
(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.
0
3 weeks ago
#6
(Original post by Sasha Shasha)
(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.
Here you've got to take into account that youre looking at a sample of 9 phone calls. So what is the distribution now for T??

If we are interested in calls within 1 standard deviation, then we seek
0
Thread starter 3 weeks ago
#7
Can you explain me further on that , im still confused

Thank you
0
3 weeks ago
#8
(Original post by Sasha Shasha)
Can you explain me further on that , im still confused

Thank you
Well, have you covered the theory on normal distribution regarding samples? If we have a normally distributed population with some mean and standard deviation, and then we take some sample of this population, then the sample will also follow normal distribution with the same mean however its standard deviation will be slightly different.

Are you aware of this, and how the sd must be slightly different ??
0
Thread starter 3 weeks ago
#9
Nop , can you explain me further ?
0
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