6.63 A vehicle driver gauges the relative speed of a preceding vehicle by the speed with which the image of the width of that vehicle varies. This speed is proportional to the speed X of variation of the angle at which the eye subtends this width.

According to Ferrani and others (Transportation Research, 18A, 1984, pp. 50–51), a study of many drivers revealed X to be normally distributed with a mean of zero and a standard deviation of 10(104) radians per second.

a What fraction of these measurements is more than five units away from zero?

b What fraction is more than 10 units away from zero?