Answer:
19. 7, 20. 4, 21. 1
Step-by-step explanation:
19. 21 = 3 x 7, 49 = 7 x 7, So they both share 7
20. 8 = 2 x 2 x 2, 52 = 2 x 2 x 13, they both have 2 x 2
21. 32 = 2 x 2 x 2 x 2 x 2, 81 = 3 x 3 x 3 x 3, they have nothing in common, so they share a 1.
