If a, b and c are in GP and a^1/x=b^1/y=c^1/z. The value of 2y is?

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

 15     12    17  10     ?    8    21     6 

If the arithmetic mean of 6, 8, 10. x, 7 is 8.the value of x will be

Direction: What should come in place of the question mark (?) in the following number series?
0.5, 2, 9, 35, 104, ?

3 4 10 33 136 ? 183

117.5, 117, 119, 111, 143, ?

3240, 540, 108, 27, ? , 4.5

Direction: What should come in the place of question mark (?) in the following number series?

24, 25, ?, 41, -8, 73