Question

# Westinghouse wants to determine the lifetime of its standard lightbulbs with an error 13 hours. Standard...

Westinghouse wants to determine the lifetime of its standard lightbulbs with an error 13 hours. Standard deviation is 120 hours. What sample size should they have to be 95% confident of the lifetime of their standard lightbulb?

a) 18.09 lightbulbs

b) 19 lightbulbs

c) 328 lightbulbs

d) 327.33 lightbulbs

Solution :

Given that,

standard deviation = = 120

margin of error = E = 13

At 95% confidence level the z is ,

= 1 - 95% = 1 - 0.95 = 0.05

/ 2 = 0.05 / 2 = 0.025

Z/2 = Z0.025 = 1.96

Sample size = n = ((Z/2 * ) / E)2

= ((1.96 * 120) / 13)2

= 327.33

Sample size = 327.33 lightbulbs

option D) is correct

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