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# Question on the Normal Model watch

1. Hello everyone.

This is my first time posting on this forum but i have always read it. I am stuck on a question on normal models. I have dont the first question, but i am stuck on b, c and d. If anyone could please explain them to me i would be greatful:

A manufacturer claims that the lifespans of the compnays photocopying machine (in months) can be described by a Normal mode N(42,7)

a) Use the 68-95-99.7 Rule to draw and clearly label the corresponding Normal Model.

I have done this question. For anyone who doesn't know what (42,7) means, 42 is the mean and 7 is the standard deviation

b) A company with several large office buildings buys 200 of these copiers. The sales man tells the boss "190 (95%) of your new photocopiers will last between ________ and _________ months". Comment

For this question i done:

(200-42)/7

The answer i got was between 22 and 23 months. But i think this is wrong becuase of what the next two questions ask

c) What is the third quartile of the copiers life spans?

d) What percent of the copiers are expected to fail before 36 months?

2. Sorry, but I can't even begin to understand what your thought process was to get (400-42)/7. (It's wrong).
3. I think the wording of your question is strange. I guess that you have been given a rule about the percentage distribution in terms of plus or minus standard deviations from the mean.

Does this graph help?

The mean life of a photocopier is given as 42 months. So it is not sensible to suggest that the life ranges from 22 months to 23 months. A sensible guess would be between 30 months and 60 months. To get the answer you need to consider mean and standard deviation.
4. hmm ok. Do you know a formula which i can find the answer?
5. Read the small print underneath the graph.

What level is this?
6. It is two standard deviations away from the mean.

Standard deviation = 7
7. In your OP you said standard deviation was 7 however you have also written N(42,7)

The 7 should be the variance which means the standard deviation is

As I said before your question looks a bit strange. Where has the question come from?
8. So what is the next step? I would have 56 and 28 which are both 2 sd away from the mean.

Thanks im really stuck..
9. An old university paper
10. Your first answer of 22 and 23 weeks leads me to believe you have no idea what you are doing.

Have you done S1 ?

Do you have the answers to this university paper?
11. I have found an interesting piece of information. How are we supposed to know which to use?

12. I have found a few clips on youtube which might be helpful.

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Updated: July 21, 2009
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