Answer
$$2,3,4,5,6,7,8,10,11,12,13,14,15,16,17,18,19,20$$
Work Step by Step
The bar means that we choose all of the values not in B between 1 and 20. Thus:
$$2,3,4,5,6,7,8,10,11,12,13,14,15,16,17,18,19,20$$
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