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The combined city/highway fuel economy of a 2016 Toyota 4Runner 2WD 6-cylinder 4-L automatic 5-speed using regular gas is a normally distributed random variable with a range 20 mpg to 24 mpg

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The combined city/highway fuel economy of a 2016 Toyota 4Runner 2WD 6-cylinder 4-L automatic 5-speed using regular gas is a normally distributed random variable with a range 20 mpg to 24 mpg.
(a) Estimate the standard deviation using Method 3 (the Empirical Rule for a normal distribution) from Table 8.11. (Round your answer to 4 decimal places.)
 Standard deviation          .6667 
.6667 Correct
(b) What sample size is needed to estimate the mean with 90 percent confidence and an error of ± 0.25 mpg? (Enter your answer as a whole number (no decimals). Use a z-value taken to 3 decimal places in your calculations.)