On an exam, a class ended with a mean raw score of 80 with standard deviation of 10. The distribution of grades in the class was approximately normal. If the teacher wants to give "A" to the top 25th percentile (top 25%) of the class, what is the lowest raw score we could get and still get an "A" on the exam?

- anonymous

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- dan815

|dw:1373527655365:dw|

- anonymous

standard deviation of 10

- dan815

lol i mean how many deviation points wud u have to travel, to be left with only 25% area on the right side

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## More answers

- dan815

look up the probability distribution graph numbers

- anonymous

Sorry sir, I could not follow you. Could you please explain

- dan815

https://www.google.ca/search?gs_rn=17&gs_ri=psy-ab&tok=tEH_WKdK4z3EM3ono5y2dg&pq=probability+distribution&cp=27&gs_id=35&xhr=t&q=probability+distribution+chart&bav=on.2,or.r_qf.&bvm=bv.48705608,d.aWc&biw=1280&bih=897&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=2GneUbvnMYWTyAH8HA#facrc=_&imgdii=_&imgrc=N1qZOwVnVPP-9M%3A%3B1jQwC4INkQn0OM%3Bhttp%253A%252F%252Fmathbits.com%252FMathBits%252FTISection%252FStatistics2%252FlabeledChart2.jpg%3Bhttp%253A%252F%252Fmathbits.com%252FMathBits%252FTISection%252FStatistics2%252Fnormaldistribution.htm%3B721%3B381

- dan815

look at that and see at what standard deviation you have to be to be left with only 25% area on the right side

- dan815

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- anonymous

Is it 21% with 0.8 standard deviation?

- anonymous

It is only one standard deviation point above the mean is to be covered. Isn't it? Other please explain

- dan815

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- dan815

how much more deviation does she have to go to cover upto 75%

- dan815

look at that chart and figure it out

- dan815

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- dan815

its somewhre inbetween 0.5 and 1

- dan815

id say around 0.75

- dan815

so 0.75 of 10 = ? 7.5 marks

- dan815

then 87.5 should be the marks she needs to set it at

- dan815

im just estimating values here, im sure u got some formulas to calculate the exact z values and stuff

- dan815

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- anonymous

This Z values I am not aware sir. I am just browsing about this

- dan815

this will give u more accurate vlaue

- dan815

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- dan815

|dw:1373533886859:dw|

- dan815

|dw:1373533895795:dw|

- dan815

|dw:1373533946389:dw|

- dan815

|dw:1373533966949:dw|

- dan815

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- anonymous

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- anonymous

what is this equation you have used

- dan815

5.6/15

- dan815

because

- dan815

look at the chart

- dan815

at 0.5 deviation on right how much percent have you covered?

- anonymous

and how did you get 0.37

- dan815

do you know the answer already

- anonymous

????? 5*6/15=0.37 ????

- dan815

whats the answer supposed to be

- dan815

its 5.6 not 5 * 6

- dan815

i cant help you if u arent answering my questions

- anonymous

No. Idon't know the answer

- dan815

then dont bother with stuff

- dan815

first understand the graph

- anonymous

I want to know more about this Z score

- anonymous

Ok sir

- dan815

- dan815

the graph is pretty self explanatory and questions about it?

- dan815

any questions?

- anonymous

just a second sir,

- anonymous

Now it is clear. But one question. May I ask

- anonymous

Why did you divide by 15. It is only 14 parts. Right

- anonymous

Sorry. Now I have fully understood.

- anonymous

It is not the 14 parts. It is the area of that part.

- anonymous

Thank you very much explaining in detailed manner

- anonymous

Thanks for your patience and analysing for a lehman.

- anonymous

Mean 80 standard deviation 10 z=-0.67
X=73.3

- anonymous

mine answer is 73.3 marks min to get A

- anonymous

you have to take Z value -0.67

- dan815

musadiq no try taking 0.67 Z

- anonymous

well table value is 0.67 that's lies at left side !

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