8
Eight would be the next number.
+6 -5 is the equation. 12 is the answer.
21: (3)+3=(6)+4=(10)+5=(15)+6=(21)
10
The next number is 10. The pattern is -7, -6, -5, -4.
The pattern is +5, +6, +7, so the next number will be 23 + 8 or 31.
8
8
8
8
6. 5-3-1 10-8-6
For an integral sequence the next number is 6 (add 5 subtract 7 pattern).