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Confusinf mean and standard deviation

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by B166418 » Sat Nov 24, 2012 9:29 am
7.51 8.22 7.86 8.36
8.09 7.83 8.30 8.01
7.73 8.25 7.96 8.53


A vending machine is designed to dispense 8 ounces of coffee into a cup .After a test that recorded the ounces of coffee in each of thousands cup dispensed by vending machine ,the 12 listed amount in ounces ,were selected from data .if the 1000 recorded amount have a mean of 8.1 ounces and standard deviation of 0.3 ounce,how many of 12 listed amounts are within 1.5 standard deviation of the mean .

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by Brent@GMATPrepNow » Sat Nov 24, 2012 9:33 am
B166418 wrote:7.51 8.22 7.86 8.36
8.09 7.83 8.30 8.01
7.73 8.25 7.96 8.53


A vending machine is designed to dispense 8 ounces of coffee into a cup .After a test that recorded the ounces of coffee in each of thousands cup dispensed by vending machine ,the 12 listed amount in ounces ,were selected from data .if the 1000 recorded amount have a mean of 8.1 ounces and standard deviation of 0.3 ounce,how many of 12 listed amounts are within 1.5 standard deviation of the mean .

4
6
9
10
11
If the Standard Deviation is 0.3 ounces, then 0.3 ounces represents 1 unit of standard deviation. Similarly, 0.6 ounces represents 2 units of standard deviation, 0.15 ounces represents 0.5 units of standard deviation, and so on.

If the mean is 8.1 ounces, then we say that 8.4 ounces is 1 unit of standard deviation above the mean (since 8.1 + 0.3 = 8.4), and we say that 7.8 ounces is 1 unit of standard deviation below the mean (since 8.1 - 0.3 = 7.8)

We want to know how many measurements in the list are within 1.5 standard deviations of the mean

Well, using the above logic, 0.45 represents 1.5 units of standard deviation.

So, 1.5 units of standard deviation below the mean equals 7.65 (8.1 - 0.45 = 7.65)
Similarly, 1.5 units of standard deviation abovethe mean equals 8.55 (8.1 + 0.45 = 8.55)

So, any measurement that is between 7.65 ounces and 8.55 will be within 1.5 standard deviations of the mean.

In the given list of measurements, the following meet this requirement:

7.51 8.22 7.86 8.36
8.09 7.83 8.30 8.01
7.73 8.25 7.96 8.53

So the answer is eleven

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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