Question 877478
The critical value for a 90% confidence interval is z = 1.645 (you use a calculator or <a href="http://3.bp.blogspot.com/_5u1UHojRiJk/TEdJJc6of2I/AAAAAAAAAIE/Ai0MW5VgIhg/s1600/t-table.jpg">table</a> to find this)


Given


xbar = 70
z = 1.645 (see above)
sigma = 5
n = 82

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Lower Bound 


L = xbar - z*sigma/sqrt(n)
L = 70 - 1.645*5/sqrt(82)
L = 69.0917006980343
L = 69.09

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Upper Bound 


U = xbar + z*sigma/sqrt(n)
U = 70 + 1.645*5/sqrt(82)
U = 70.9082993019657
U = 70.91


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The confidence interval (L,U) is <font color="red">(69.09, 70.91)</font>