Solution:

4

No Of circles in the number.

Solution:

52

sum of consecutive prime numbers
(2+3 , 5+7 , 11+13 , 17+19 , 23+29)

Solution:

186

Explanation :
21+11*1 => 32
32+11*2 => 54
54+11*3 => 87
87+11*4 => 131
131+11*5 => 186

Solution:

4

92 - 38 = 54
5 is given and 4 is missing

Solution:

4

6*3 equals 18
7*9 equals 63
6*4 equals 24
7*6 equals 42, ? should be replaced by 4.

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:

276

6 = 11 + 21 + 31
14 = 12 + 22 + 32
36 = 13 + 23 + 33
98 = 14 + 24 + 34
276 = 15 + 25 + 35

Solution:

245

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