Consider another random variable D¯ = X¯ −Y¯ . Using the above fact, we know that the D¯ will follow a normal distribution. However, we still don’t know what the mean of that distribution is. (a) What is E(D¯)? (b) What is Var(D¯)? (c) What would it mean in terms of the problem if E(D¯) > 0?