Question

Find the numbers in d blank

Answer 1

Lets say:
Top row: a + b = 8
bottom row: c - d = 6 
left column: a + c = 13
right column: b + d = 8

As you know: a + b = 8 
Also               b + d = 8 
so from here we know a = d  

c -d = 6  then---> c - a = 6 (because a = d)  -> a = c - 6
We know a + c = 13 then
              (c - 6) + c = 13
              2c -6 =13
              2c = 19
               c = 19/2 = 9.5
             
but c - a = 6
      9.5 - a = 6
       a = 9.5 - 6 =3.5 

but a =  d = 3.5

a + b = 8
3.5 + b = 8
b = 8 -3.5 = 4.5

so a = 3.5, b = 4.5, c = 9.5, d = a = 3.5

Result:
a + b = 3.5 + 4.5 =8
c - d = 9.5 - 3.5 = 6
a + c = 3.5 + 9.5 = 13
b + d = 4.5 + 3.5 = 8

Answer 2

The missing numbers would be 12-6