18, 24, 5, 21, 27, 8, 17, 23, 4, 24, 30, ?

• consider (18,24,5)(21,27,8)(17,23,4)(24,30,?)
  in every series 2nd no increases +6 and den decreases -19
  so, ? mark will be (30-19)=11.
• 11 years agoHelpfull: Yes(33) No(2)