The life expectancy of a typical lightbulb is normally distributed with a mean of 2,000 hours and a standard deviation of 27 hours. What is the probability that a lightbulb will last between 1,975 and 2,050 hours?

I figure that since one standard deviation is 27, we are looking at 3 standard deviations away from the mean(1 less than the mean and 2 more than the mean). Is this right? Where do I go from here?

Question

The life expectancy of a typical lightbulb is normally distributed with a mean of 2,000 hours and a standard deviation of 27 hours. What is the probability that a lightbulb will last between 1,975 and 2,050 hours?

I figure that since one standard deviation is 27, we are looking at 3 standard deviations away from the mean(1 less than the mean and 2 more than the mean).  Is this right? Where do I go from here?

anonymous · Answer

The answer options are as follows:
A  0.17619
B   0.32381
C   0.79165
D  0.96784

anonymous · Answer

I have no idea how to do this, will fan and medal

mrnood · Answer

no - it's not 3sd from the mean

it's from -1 to +2sd from mean

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So you know tharea between mean and -1
you know th earea between mean and +2

so add them up 

(use 68, 95, 99.7  rule)