12 : 18 = 2 : 3 (divide both by 6)
The next number in the series 4, 12, 6, 12, 36, 18, 36
The next number in the series 4-12-6-12-36-18-36.. is 108.
36
36
(6), 12, 18, 24
24
36 72 66
36
36
6 12 9 18 15 30 27 54 51
-3 0 3 then 6 9 12 15 18 21 and so on ...