Question

А + В = 8
- +
С + D = 6
= =
8 13

Answer 1

a + b = 8 => a = 8 - b b + d = 8 => d = 8 - b, 
= d a + c = 13 => a = 13 - c c - d = 6 => d = c - 6 
 a = d c - 6 = 13 - c => 2c = 19 => c = 9.5
: A+b=8,c-d=6,a+c=13,b+d=8..
a + b = 8 => a = 8 - b b + d = 8 => d = 8 - b, 
 a = d a + c = 13 => a = 13 - c c - d = 6 => d = c - 6 
 a = d c - 6 = 13 - c => 2c = 19 => c = 9.5