The lifetimes of light bulbs of a particular type are normal

The lifetimes of light bulbs of a particular type are normally distributed with a mean of 370 hours and a standard deviation of 5 hours.What percentage of bulbs has lifetimes that lie within 1 standard deviation of the mean on either side?

Having problems trying to use excel for this question. Can you explain. thanks

Solution

Let X be the life times of light bulbs of particular type

X is N(370, 5)

From std normal distribution table within 1 std deviation = 68.26% lie.