A type of battery-operated led lights has a known mean lifetime 7.9 hours with standard deviation 0.5. Assuming that the lifetimes of these led lights have a nearly symmetric/bell-curve distribution, find the percent (%) of these led lights having lifetime between 7.9 and 8.9 hours.

47.5% of these led lights have lifetime between 7.9 and 8.9 hours.

Step-by-step explanation:

The Empirical Rule states that, for a normally distributed random variable:

68% of the measures are within 1 standard deviation of the mean.

95% of the measures are within 2 standard deviation of the mean.

99.7% of the measures are within 3 standard deviations of the mean.

In this problem, we have that:

Mean = 7.9 hours

Standard deviation = 0.5 hours

The normal distribution is symmetric, which means that 50% of the measures are below the mean and 50% are above.

Lifetime between 7.9 and 8.9 hours:

7.9 hours is the mean.

8.9 = 7.9 + 2*0.5

So 8.9 hours is two standard deviations above the mean.

Of the 50% of the measures that are above the mean, 95% are between the mean of 7.9 and two standard deviations above the mean(8.9). So

0.5*0.95 = 0.475

47.5% of these led lights have lifetime between 7.9 and 8.9 hours.