What will come in place of the question mark (?) in each of the following series?

 15     12    17  10     ?    8    21     6 

What will come in place of the question mark (?) in each of the following series?

15     ?    24    33     97    122     

What will come in place of question mark ( ? ) in the following series?
656, 352, 200, 124, 86, ? 

What will come in place of question mark ( ? ) in the following series?
13, 25, 61, 121, 205, ? 

What will come in place of question mark (?) in the following series?
8, 10, 18, 44, 124, ?