what percentage of lights burn less than 960 hours

Question

suppose the burning times of electric light bulbs approximate a normal curve with a mean of 1200 hours and a standard deviation of 120 hours.

Answer 1

Given mean (  )= 1200 hours

Standard deviation (  )  is 120 hours

Now we have to calculate percentage of lights burn less than 960 hours

As per normal distribution form 

Hence it is in the form of x - 2

In statistics the 68–95–99.7 rule, also known as the three-sigma rule or empirical rule, states that nearly all values lie within three standard deviations of the mean in a normal distribution

μ ± 1σ

= 0.682689492137086

μ ± 2σ

= 0.954499736103642

μ ± 3σ

= 0.997300203936740

So from emperical rule , the percentage of lights burn less than 960 hours is 95.64%