Question

An electrical firm manufactures light bulbs that have a length of life that is approximately normally distributed with a standard deviation of 40 hours. If a sample of 30 bulbs has an average life of 780 hours, find a 96% confidence interval for the population mean of all bulbs produced by this firm.

Answer 1

It is given that population standard deviation is = 40 hours

sample mean = 780 hours and sample size is n = 30

we have to find 96% confidence interval for the population mean, this means

Using z distribution table for critical value, we get z= 2.054

Formula for confidence interval is given as

setting the given values, we get

Therefore, required 96% confidence interval is from 765.0 to 795.0