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In a random sample of 30 tires of the same type, it is found that the average life span is 36,200 miles with a standard deviation of 3800 miles.

Find the interval about the sample mean such that the probability is 0.77 that the mean number lies within the interval.
a. 35,402-36,998
b. 31,830-40,570
c. 35,436-36,964
d. 35,367-37,033

Is it C?

  • Math -

    You are looking for the proportion that lie in the area μ ± .385 = μ ± 1.2 SD. (This information is available from a table in the back of your stats text called something like "areas under the normal distribution.")

    However, for a distribution of means, you need the standard error rather than the standard deviation.

    SE = SD/sq rt of n

    So you are looking for μ ± 1.2SE

    You should be able to calculate the values from this.

    I hope this helps. Thanks for asking.

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