試題詳解

17. Two instruments are used to measure the height, h, of a tower. The error made by the less accurate instrument is normally distributed with mean 0 and standard deviation 0.0056h. The error made by the more accurate instrument is normally distributed with mean 0 and standard deviation 0.0044h.
 The errors from the two instruments are independent of each other.
Calculate the probability that the average value of the two measurements is within 0.005h of the height of the tower.
(A) 0.38
(B) 0.47
(C) 0.68
(D) 0.84
(E) 0.90