The lifetime in hours of a certain make of batteries follows a normal distribution with mean 160 and standard deviation 30.

Question

Calculate the interval, symmetrical about the mean within which 75% of the battery lifetimes lie.

Answer 1

The following applet takes the place of your tables or charts traditionally used for this kind of work
http://davidmlane.com/normal.html

click on : value from an area
enter 0.75 in "area"
enter 160 in "mean"
enter 30 in "SD"

under "results" click "between"
click on "recalculate" to get

125.5 to 194.5