Posted by **MARIA** on Monday, January 9, 2012 at 10:47pm.

A new extended-life light bulb has an average service life of 750 hours, with a standard deviation of 50 hours. If the service life of these light bulbs approximates a normal distribution, about what percent of the distribution will be between 600 hours and 900 hours?

- statistics -
**MathGuru**, Tuesday, January 10, 2012 at 4:27pm
Use z-scores.

Formula:

z = (x - mean)/sd

Find two z-scores, using 600 for x and also 900 for x. Mean = 750 and sd = 50.

Once you find the two z-scores, use a z-distribution table to determine your probability between the two z-scores.

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