Battery manufacturers compete on the basis of the amount of time their products last in cameras and toys. A manufacturer of alkaline batteries has observed that its batteries last for an average of...

Battery manufacturers compete on the basis of the amount of time their products last in cameras and toys. A manufacturer of alkaline batteries has observed that its batteries last for an average of 6.9 hours when used in a toy racing car. The amount of time is normally distributed with a standard deviation of 2.1 hours.

a. What is the amount of time below which only 10% of all a battery lasts?