Ответ:

4

6, 9

8, 12, 16

10, 15, 20, 25

12, 18, 24, 30, 36

14, 21, 28, 35, 42, 49

16, 24, 32, 40, 48, 56, 64

18, 27, 36, 45, 54, 63, 72, 81