posted by Blerp .
An electrical firm which manufactures a certain type of bulb wants to estimate its mean life. Assuming that the life of the light bulb is normally distributed and that the standard deviation is known to be 40 hours, how many bulbs should be tested so that we can be 90 percent confident that the estimate of the mean will not differ from the true mean life by more than 10 hours?
n = [(z-value * sd)/E]^2
...where n = sample size, z-value will be found using a z-table to represent the 90% confidence interval, sd = 40, E = 10, ^2 means squared, and * means to multiply.
Plug the values into the formula and finish the calculation. Round your answer to the next highest whole number.