Thread: Confidence Interval and Standard Deviation

I can't get these ones at all x.x please help


ABC College Bookstore sells Ti83 calculators. The random number of calculators sold per day along with the probabilities is:

# of calculators: 1, 2, 3, 4, 5
Probability: 1/8, 1/6, 1/2, 1/12, 1/8

What is the standard deviation for the number of calculators sold?


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Find a 95% confidence interval for the mean weight of bass caught in El Capitan lake if a sample of 300 fish had a mean weight of 77 ozs. with a standard deviation of 12 ozs.



I've been trying to figure these out for a while and still can't get it right.

Originally Posted by Phil32686

I can't get these ones at all x.x please help


ABC College Bookstore sells Ti83 calculators. The random number of calculators sold per day along with the probabilities is:

# of calculators: 1, 2, 3, 4, 5
Probability: 1/8, 1/6, 1/2, 1/12, 1/8

What is the standard deviation for the number of calculators sold?


---

Find a 95% confidence interval for the mean weight of bass caught in El Capitan lake if a sample of 300 fish had a mean weight of 77 ozs. with a standard deviation of 12 ozs.



I've been trying to figure these out for a while and still can't get it right.

Q1 Use . If you need more help, please show your working and say where you get stuck.

Q2 Read this thread (specifically, reply #2): http://www.mathhelpforum.com/math-he...-interval.html

You will find many questions on confidence intervals (with helpful replies) at MHF by using the Search tool (top right).

Originally Posted by mr fantastic

Q1 Use . If you need more help, please show your working and say where you get stuck.

Q2 Read this thread (specifically, reply #2): http://www.mathhelpforum.com/math-he...-interval.html

You will find many questions on confidence intervals (with helpful replies) at MHF by using the Search tool (top right).

For the first one, would I plug in probability for E and # of calculators for X?

= (1/8)(1^2) - [(1/8*1)^2] + etc...

And do that for all of the # of calculators and probabilities for each?

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For the Confidence Interval question, I got

75.6421 < u < 78.3579

Originally Posted by Phil32686

For the first one, would I plug in probability for E and # of calculators for X?

= (1/8)(1^2) - [(1/8*1)^2] + etc...

And do that for all of the # of calculators and probabilities for each?

[snip]

You calculate these things using the usual definitions:

E(X) = (1)(1/8) + (2)(1/6) + ....

E(X^2) = (1^2) (1/8) + (2^2) (1/6) + ....