7.18 In a study of particulate pollution in air samples over a smokestack, X1 represents the amount of pollutant per sample when a cleaning device is not operating, and X2 represents the amount when the cleaning device is operating. Assume that (X1, X2) has the joint probability density function

= e1 0 … x1 … 2, 0 … x2 … 1, 2x2 … x1 0 elsewhere The random variable Y X1 X2 represents the amount by which the weight of pollutant can be reduced by using the cleaning device.
a Find E(Y ) and V(Y ).
b Find an interval in which values of Y should lie at least 75% of the time.