Question

15,29,56,108,205,400.......what is the next.

a.770
b.756
c.758
d.768

Answer 1

The 5th no. must be 208

1 x 15 - 0 x 0 = 15, 
2 x 15 - 1 x 1 = 29, 
4 x 15 - 2 x 2 = 56, 
8 x 15 - 3 x 4 = 108, 
16 x 15 - 4 x 8 = 208,
32 x 15 - 5 x 16 = 400,

64 x 15 - 6 x 32 = 768