Solution:

20 (double of previous)

Solution:

5

(8+7) * 5 => 75

Solution:

43

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.