The times per workout an athlete uses a stair climber are normally distributed with a mean of 20 minutes and a standard deviation of 5 minutes. What percentage of athletes use the stair climber for less than 18 mi utes

Apinhole camera is made out of a light-tight box with a piece of film attached to one side and pinhole on the opposite side. the optimum diameter d(in millimeters) of the pinhole can be modeled by d=1.9[ (5.5 x 10^-4) l) ] ^1/2, where l is the length of the camera box (in millimeters). what is the optimum diameter for a pinhole camera if the camera box has a length of 10 centimeters