Let the missing terms of the series be x1 and x2.Thus, the sequence 20, 20, 19, 16, 17, 13, 14, 11, xv x2 is a combination of two series :I. 20, 19, 17, 14, x1 and II. 20, 16, 13, 11, x2The pattern in I is - 1, - 2, - 3,......So, missing term, x1 = 14 - 4 = 10.The pattern in II is - 4, - 3, - 2,......So, missing term, x2 = 11 - 1 = 10.