The mean life of a particular brand of light bulb is 1200 hours. if you know that at about 95% of this brand of bulbs will last between 800 and 1600 hours, then what is the standard deviation of the light bulbs' life?

204
   First, lookup a standard normal table and see what the z-score is for 0.025 (one half of 100% - 95%) to allow for equal sized tails. You should find that the z-score is 1.96. That means that 95% of the time, the value should be within 1.96 standard deviations of the mean. Now let's calculate the standard deviation. 800 is 800 - 1200 = -400 to the left of the mean of 1200. 1600 is 1600 - 1200 = 400 to the right of the mean of 1200. So we are an equal distance of 400 on both sides of the mean. And we know from the z-score of 1.96, that we're 1.96 standard deviations from the mean. So a little division will give us the standard deviation. Which is: 400 / 1.96 = 204.0816327
   So the standard deviation of the light bulbs is 204

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