Posted by **Blerp** on Friday, November 16, 2012 at 11:35am.

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?

7

44

8

62

87

- Statistics -
**MathGuru**, Friday, November 16, 2012 at 7:15pm
Formula:

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.

- Statistics -
**Anonymous**, Sunday, March 30, 2014 at 3:09pm
44

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