What mathematical symbol can be placed between 5 and 9, to get a number greater than 5 and smaller than 9? 

Hence, the missing number iss 25.

The missing number in the given number puzzle is "78".

Here the logic is the numbers are from 86 - 91 when we read upside down.

The required number 100.

It is not an odd number and is not higher than 100 and greater than 90.

When we subtract 100 from 100, we get nothing i.e, 0.

The logic in the given puzzle is 

4 + 4 = 4 * 4+1 = 4 * 5 = 20

5 + 5 = 5 * 5+1 = 5 * 6 = 30

6 + 6 = 6 * 6+1 = 6 * 7 = 42

7 + 7 = 7 * 7+1 = 7 * 8 = 56

Similarly

9 + 9 = 9 * 9+1 = 9 * 10 = 90.

Let the value to be find be 'x'.

 
Sum of individual column value = 14

 
C1 = 7 + 2 + 1 + 4 = 14
C2 = 3 + 8 + 1 + 2 = 14
C3 = 6 + 5 + 2 + 1 = 14

C4 = 2 + 4 + 4 + x = 14

x = 14 - 10

x = 4

Hence, the missing number is '4'.

1 + 1 = 121 = 1x1 1+1 1x1

1 + 2 = 134 = 1x1 1+2 2x2

2 + 2 = 444 = 2x2 2+2 2x2

3 + 2 = 954 = 3x3 3+2 2x2

3 + 3 = ?   

Similarly, 3x3 3+3 3x3 = 969

In the given number puzzle,

Sum of Row 1 = 14

Sum of Row 2 = 23

Sum of Row 3 = 13 + ?

Sum of Column 1 = 16 

Sum of Column 2 = 23 

Sum of Column 3 = 11 + ?

Diagonal 1 sum = 17

Diagonal 2 sum = 14 + ?

From the above analysis,

Sum of Row 2 = Sum of Column 2 = 23

Sum of Row 1 = Sum of Column 3 = 14 = 11 + ?    .............(1)

Sum of Row 3 = Sum of Column 1 = 16 = 13 + ?    .............(2)

Diagonal 1 sum = Diagonal 2 sum = 17 = 14 + ?    .............(3)

Hence,

From (1), (2) & (3)

? = 3

Here in the given number puzzle, the logic is

column 2 x column 3 = column 1

3 x 3 = 9

2 x 3 = 6

Similarly,

4 x ? = 8

=> ? = 8/4 = 2.