Solution:

6

sum of opposite number is 21

Solution:

-4

298 less 209 is 89
209 less 129 is 80
129 less 58 is 71
58 less -4 is 62
The result of each subsequent equation is 9 less than the previous one.

Solution:

240

40 { diff 20 + 12 =>32 add to it}
72 { diff 32 + 12 =>44 add to it}
116 { diff 44 + 12 =>56 add to it}
172 { diff 56 + 12 =>68 add to it}
240

Solution:

245

space them properly as 21 24 27 30 33 36 39 42 45 ?

Solution:

-20

Its a alternate series riddle
60 40 20 0 (?)
40 43 46 49 (+3)

Solution:

17

It is the sum of the two digits(9 + 8) in opposite quadrant.

Solution:

5

23-2=21
21+3=24
24-5=19
19+7=26
26-11=15
15+13=28
28-17=11
11+19=30
30-23=7
7+29=36
36-31=5


(2,3,5,7,11,13,17,19,23,29,31,37,41,43... are Prime Numbers)

Solution:

24

write the numbers in ascending order in words as
onE (Position of 'E' in alphabet is 5)
twO (Position of 'O' in alphabet is 15)
threE (Position of 'E' in alphabet is 5)
fouR (Position of 'R' in alphabet is 18)
and so one

Solution:

20(Twenty) : when you write them , these are only numbers having vowel 'E'

Solution:

The answer is 1

6 {6^1}
25 {5^2}
64 {4^3}
81 {3^4}
32 {2^5}
1 {1^6}