Letter of credit is opened at the request of whom among

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

16, 16, ?, 96, 384, 1920

150     15      3      ?      0.36       0.18

2807, 1400, 697, 346, 171, 84, 41, ?

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

100, 81, 64, 51, 40, ?

24, 12, 18, 45, ?,  708.75

8, 9, 125, ?, 1331, 169 

16, 9, ?, 114.25, 750.625, 6396.3125

60, 100, 160, 240, ?, 460

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

15, 90, 450, ?, 5400, 10800 

4, 9, 24, 69, ?, 609