I still need help please

#1
Ok, so I am still working on this freaking problem. Can someone help me please. It states that the scenario is normally distributed with a mean price of $36 and SD of $3.50. The company wants to sell in the middle 70% range so I need to find the min and max price. I really can't seem to figure out how to do it. The whole "middle of the 70% range is screwing me up I think. I know I need to find a z value because I am given the probability so I need to find X. So, what I did was took the 70% and divided it in half for 35%. Then, I found the z value corresponding most closely to 0.350 to be 1.04. I filled this in my equation of z=x-u/SD and found +/- 3.64 therefore I said the min selling price should be 36-3.64 and the max should be 36+3.64. Can someone please advise if I am on the right path? I cannot seem to find a similiar problem in my text where we deal with the middle of a range.

Thanks in advance,

Denise :)
 
#4
BestNurse,

To be frank, I'm not really sure what the question is asking when it says: "in the middle 70% range".

However, we do have:

mu = $36
sigma = $3.50

and what you tried to compute sounds like the 35th percentile and 65th percentile.

However, your value for z should then be 0.39

If you look in the table you should see for 0.39 the value 0.6517, and the value for -0.39 is 0.3482 (which is about 1 - 0.6517)

Not sure what you did.

David
 
#5
Anyone else have an opinion on this question? I am still not sure of how to work it out and I am pretty certain what I have done is not correct.

Thanks,

Denise
 
#6
Anyone else have an opinion on this question? I am still not sure of how to work it out and I am pretty certain what I have done is not correct.

Thanks,

Denise
Denise,

What could help is to have the whole problem statement in context. Does it appear in a textbook? If so, the name of the chapter, and the section can help to figure out what the problem is asking you to apply.

Posting the whole problem statement verbatim may help to understand what is going on.

David
 
#7
and what you tried to compute sounds like the 35th percentile and 65th percentile.
i don't think it's 35th and 65th percentile (how would it make it middle 70%?), rather 50-70/2=15 and 50+70/2=85 th percentiles. And she looked up the z value for 85th percentile. I think she used the table below when she said "I found the z value corresponding most closely to 0.350 to be 1.04".

take a look at this table:

http://www.science.mcmaster.ca/psychology/poole/z-table2.jpg
 
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#8
Ok, I am still confused. I did email my tutor but he hasn't responded yet and I would really like to get this question finished so I can send my assignment in. Like I said, when it says in the middle 70 range, I am not sure how to deal with that. That is what has created the problem for me. I know I have to do something with that 70% obviously. The part in the book that I am working on has to deal with finding the X value when you are given the probability and all the other questions were about finding a value such as "more than 70%" or less than a percentage. I was able to figure those out because I had to use the 50% mark (or the mean) and either subtract or add the z value. This one has me confused because I am not sure what to do with the "middle 70%" and don't know how to find the min and max price. Any other thoughts?

Thanks,

Denise
 

CowboyBear

Super Moderator
#11
Ok, so I am still working on this freaking problem. Can someone help me please. It states that the scenario is normally distributed with a mean price of $36 and SD of $3.50. The company wants to sell in the middle 70% range so I need to find the min and max price. I really can't seem to figure out how to do it. The whole "middle of the 70% range is screwing me up I think. I know I need to find a z value because I am given the probability so I need to find X. So, what I did was took the 70% and divided it in half for 35%. Then, I found the z value corresponding most closely to 0.350 to be 1.04. I filled this in my equation of z=x-u/SD and found +/- 3.64 therefore I said the min selling price should be 36-3.64 and the max should be 36+3.64. Can someone please advise if I am on the right path? I cannot seem to find a similiar problem in my text where we deal with the middle of a range.

Thanks in advance,

Denise :)
This is my reading of the question:

M = 36
SD = 3.5
Find 70% confidence interval.

Shouldn't be too tough. You can check your answer using the bottom calculator on this page:

http://davidmlane.com/hyperstat/z_table.html

Edit: Yep, looks like you have the right answer, within rounding error.
 
#12
This is my reading of the question:

M = 36
SD = 3.5
Find 70% confidence interval.

Shouldn't be too tough. You can check your answer using the bottom calculator on this page:

http://davidmlane.com/hyperstat/z_table.html

Edit: Yep, looks like you have the right answer, within rounding error.
Yes, if this is what is meant by 70% range then this makes sense.

I think this sort of makes sense within the context of the problem because maybe sometimes demand for the product is low, and so they decrease the price, and sometimes demand for the product is high, and so they increase the price (but they don't want to increase or decrease the price beyond certain bounds)

David
 
#13
Thanks for the help. I was thinking I was on the right path but do not have an example of this type of question in my text so I just wanted to make sure before I send my assignment in.

Denise