Question

If the population standard deviation of the lifetime of washing machines is estimated to be 900 hours, how large a sample must be taken in order to be 90% confident that the margin of error will not exceed 100 hours?

Select one: a. 219.18 b. 220 c. 14.8 d. 15

Answer 1

solution:- option b. 220

explanation:-

given that standard deviation = 900 , margin of error E = 100

the value of 90% confidence from z table is 1.645

sample size

formula

=> n = (z*standard deviation/E)^2

=> n = (1.645*900/100)^2

=> n = 219.188

=> n = 220 (rounded)