Let the missing terms of the series be x_{1} and x_{2}.

Thus, the sequence 20, 20, 19, 16, 17, 13, 14, 11, xv x2 is a combination of two series :

I. 20, 19, 17, 14, x_{1} and II. 20, 16, 13, 11, x_{2}

The pattern in I is - 1, - 2, - 3,......So, missing term, x_{1} = 14 - 4 = 10.

The pattern in II is - 4, - 3, - 2,......So, missing term, x_{2} = 11 - 1 = 10.