1500 is 2 standard deviations (500 * 2 = 1000) below the mean of 2500.
95% = both directions from the mean, but you are only considering the lower end, so?
If light bulbs have lives that are normally distributed with a mean of 2500 hours anda standard deviation of 500 hours, use the 68-95-99.7 rule to approximate the percentage of lightbulbs having a life less than 1500.
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