Need help with Stats probability question

minagile

Hi, I have problem to solve this question.

A certain brand of cattle feed comes in bags marked 40 kg. The grain company claims that the bags contain an average of 40 kg of grain and that the standard deviation of the weights is 1.8 kg. Furthermore, it is known that the weights are normally distributed.
(a) If the company’s claim is true, what is the probability that the weight of a randomly selected grain bag will be 39 kg or less?
(b) What is the probability that the mean weight of a 10 randomly selected grain bags will be 39 kg or less?
(c)If you bought a bag of grain and it weight 39 kg, would you consider this to be evidence that the company’s claim is incorrect?
(d) If you bought 10 bags of grain and the average weight was 39 kg, would you consider this to be evidence that the company’s claim incorrect?

Please help me with this question. Thanks a lot!!!!!

bnt

Um ... you're basically asking us to give you a course on basic statistics. Don't you have any materials to study? Or do you think we're going to do your assignment for you? That's not what we do here - see the Charter Sticky.

Hint: in part (a), use the standard equation to convert the given data to the Z domain, so you can look it up in the table:
[latex]Z=\frac{X - \mu}{\sigma}[/latex]

Jake Rugby Walrus666

39kg!
That is not what you paid for. You paid for 40.
Demand a refund.

minagile

um....I wasn't just want you solve the question for me........:eek:

anyway I did the calculation for this question, but I am not sure I use the right method for part (c) & (d)????

(c) If you bought 1 bag of grain and it weight 39 kg, would you consider this to be evidence that the company’s claim is incorrect?

Answer:



(d) If you bought 10 bags of grain and the average weight was 39 kg, would you consider this to be evidence that the company’s claim incorrect?

Answer:



my question is I chose 95% confident interval, is that appropriate for (c) & (d), i was thinking may be I should use 90% for (c) and 99% for (d)? is this method my for question was right at all?

Please help, thanks a lot. I am really stuck.

marglin

ya 95% is fine to use. i'm pretty sure i did this module in first year. Don't worry, in this and nearly all subsequent stats modules you do, use 95% significance levels unless otherwise stated in the question, i think lecturers should be happy enough with that. looks right anyway

minagile

thank you!