Question

What would the common difference be for this sequence: 10, 8, 5, 1, -4

Answer 1

Well,
$10-8=2 \\ 5-4=1 \\ 1-4= (-4) \\ 10 and 8 are not common with 5,1, and -4$
so, all 5,1, and -4 are common because they can be added and subtracted by the same number(4) and get the next number.

Answer 2

10,      8,        5,      1,       -4
    x - 2,  x - 3,  x - 4,  x - 5
         x - 1,    x - 1,   x - 1