Answer:
17, 19
Explanation:
15 is composite (5, 3) (15, 1)
16 is composite (8, 2) (4, 4) (16, 1)
17 is prime (17, 1)
18 is composite (9, 2) (6, 3) (18, 1)
19 is prime (19, 1)
20 is composite (2, 10) (4, 5) (20, 1)
17 and 19
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