19.
Ten.
43
If you mean: 9, 19, 39, 69 then the next number in the sequence is 109
23 Prime numbers are odd numbers and 21=3x7
31
35
19.
Ten.
67
43
If you mean: 9, 19, 39, 69 then the next number in the sequence is 109
27
27
23 Prime numbers are odd numbers and 21=3x7
Crib! I haven't played in decades. The number 19 comes to mind, but it was so long ago. not played for awhile but it is 19
1.77 because 1.6=1.60 and 1.7=1.70