Solution:

To make things easier, let us just denote each box with the help of R number and C number i.e. the row number and the column number.

The rule that has been used here is:
R4C1 = (R1C1 * R3C1) - R2C1 = (5 * 2) - 8 = 2
R4C2 = (R1C2 * R3C2) - R2C2 = (7 * 4) - 10 = 18
R4C3 = (R1C3 * R3C3) - R2C3 = (6 * 5) - 12 = 18

Using the same rule for the last box:
R4C4 = (R1C4 * R3C4) - R2C4 = (4 * 9) - 17 = 19

Thus 19 is the missing number.

Solution:

The number will be 30.

In the first circle
(5 + 4) * 2 = 18

In the third circle
2. (12 + 8) * 3 = 60

In the fourth circle
3. (3 + 9) * 4 = 48

Similarly In the fifth circle
4. (4 + 2) * 5 = 30

Solution:

125

Explanation:
The trick that has been used here is that all the corner digits have been reversed and then added. The sum is than placed in the center.

In the first square:
13 + 32 + 11 + 34 = 90

In the second square:
21 + 19 + 27 + 16 = 83

In the third square:
33 + 33 + 22 + 26 = 125

Thus the missing number is 125

Solution:

The solution is given in the attached image.

Solution:

The number 20 fits in place of question mark.

This is because (number in first column) / (number in second column) * 8 = number in third column.

Here (left number / middle number) * 8 = right number
E.g. (35 / 20) * 8 = (7/4) * 8 = 7 x 2 = 14
The same process can be used to find out the missing digit.

Solution:

There is no mention if you can use sign or not and thus the only possible way to get this is as follows:

(15 - 9) + (13 - 7) + (7 - 1) + (9 - 1) + (13 - 9)

If you solve inside the brackets, you will get the following equation
6 + 6 + 6 + 8 + 4
Adding all these numbers, you will get 30 which is the required sum.

Solution:

60

Explanation:
1 + 5 = 12
But it can be true only if we multiply both digits with 2 and then add:
1*2 + 5*2 = 12

2 + 10 = 24
But it can be true only if we multiply both digits with 2 and then add:
2*2 + 10*2 = 24

3 + 15 = 36
But it can be true only if we multiply both digits with 2 and then add:
3*2 + 15*2 = 36

Similarly,
5 + 25 can be true only if we multiply both digits with 2 and then add:
=> 5*2 + 25*2 = 60

Therefore, the missing number is 60.

Solution:

Q

Explanation:
First Square:
6 + 4 + 4 = 14
N is the 14th letter.

Second Square:
4 + 1 + 7 = 12
L is the 12th letter.

Third Square:
5 + 6 + 10 = 21
U is the 21st letter.

Fourth Square:
14 + 2 + 1 = 17
Q is the 17th letter.

Solution:

100

Series of number that do not contain albhabet 'T'

Solution:

The last missing throw will have 3 as the result. This is because, with each pair of the throws you can find that the total sum is increasing by three and then decreasing by one. Therefore the missing throw be 3.