Question 1083275
Use a <a href="http://www.sjsu.edu/faculty/gerstman/StatPrimer/t-table.pdf">stats table</a> to find that the critical value is roughly z = 1.645
To find this value, locate "90%" at the bottom (in the Confidence Level section) and the value just above this entry is 1.645


The population standard deviation sigma is given to be sigma = 1.25


The margin of error desired is 0.25, so E = 0.25


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We will plug z = 1.645, sigma = 1.25 and E = 0.25 into the <a href="https://www.ltcconline.net/greenl/courses/201/estimation/ciprop.htm">formula</a> below to find n.


n = ( (z*sigma)/E )^2


n = ( (1.645*1.25)/0.25 )^2


n = ( 2.05625/0.25 )^2


n = ( 8.225 )^2


n = 67.650625


n = 68 ... round up to the nearest whole number


Min Sample Size: n = 68


So this means that she can use a sample size of 68 or larger.


She should take <font color=red>at least 68</font> measurements to ensure that the radius of the donuts is within +- 0.25cm.