6.) 7, 10, 15, 22, 31, .. Neither (but increasing difference, 3, 5, 7, 9, ... is arithmetic with d=2)
7.) 1, -3, 5, -7, .. Neither (but differences alternate -4, +8, -12, ...)
8.) 80, 40, 20, 10, .. Geometric: ratio=1/2
9.) 20, 30, 36, 42, .. Neither
10.) 100, -50, 25, -12.5, .. Geometric: ratio=-1/2