Martin suspects that people carry less cash with them than they used to because of credit cards and other cashless payment systems. He decides to work on inventing a new system for people to use in place of cash that would be more user friendly and safe.
Martin conducted a survey to help convince investors to back the development of his new cashless system. One afternoon at the local mall, Martin surveyed a random sample of 95 people and asked the question, How much cash are you carrying in your wallet? The mean of his data is $8.00 with a standard deviation of $2.50. Assume the data is normally distributed. Based on this information, answer the following questions.
1-What is the probability that a person who was surveyed has less than $5 in his or her wallet?
2-What is the probability that a person who was surveyed has between $9 and $10 in his or her wallet?
3-Now, Martin can reasonably guess that the standard deviation for the entire population of people at the mall during the time of the survey is $1.50. What is the 95% confidence interval about the sample mean? Interpret what this means in the context of the situation where 95 people were surveyed and the sample mean is $8. Use the information in this resource to help construct the confidence interval.
4-Would the interval found in part C increase, decrease, or remain the same if the confidence level desired were 99%? State your reasoning.
5-Would the interval found in part C increase, decrease, or remain the same if fewer than 95 people were surveyed? Justify your answer.
I also offer a calculator aid to help although this may not be what you need.
Look in thew sticky topics on the left and open the post called 'reference material'
Now scroll down to the heading "Other Calculators"
There is one there called "z score probabilities - this is great"
Try that and you should be able to answer your questions I think.
There are many great things in the sticky topic posts, especially in 'reference material' one.