A question about coffee....

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DFritschy: Junior | Next Rank: 30 Posts; Posts: 28; Joined: Thu Jun 21, 2012 4:25 pm

by DFritschy » Mon Aug 27, 2012 2:39 pm

Can someone please show me the steps involved to solve this question?

A vending machine is designed to dispense 8 oz of coffee into a cup. Out of 1,000 cups, 12 amounts were chosen. If the 1,000 amounts have a mean of 8.1 oz and a standard deviation of 0.3 oz, how many of the 12 listed are within 1.5 standard deviations of the mean?

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

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Source: Beat The GMAT — Problem Solving | Mon Aug 27, 2012 2:39 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Mon Aug 27, 2012 3:57 pm

DFritschy wrote:Can someone please show me the steps involved to solve this question?

A vending machine is designed to dispense 8 oz of coffee into a cup. Out of 1,000 cups, 12 amounts were chosen. If the 1,000 amounts have a mean of 8.1 oz and a standard deviation of 0.3 oz, how many of the 12 listed are within 1.5 standard deviations of the mean?

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

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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