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Finding the critical value t* for a desired confidence level

You might need: Calculator,t table

Problem

Emilio took a random sample of n=12 giant Pacific octopi and tracked them to calculate their mean lifespan. Their lifespans were roughly symmetric with a mean of x¯=4 years and a standard deviation of sx=0.5 years. He wants to use this data to construct a t interval for the mean lifespan of this type of octopus with 90% confidence.
What critical value t should Emilio use?
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