В квабратах должни быть цифры

A + B = 8

+ +

C – D = 6

= =

13 8

A + B = 8, B + D = 8 => A + B = B + D => A = D;

A + C = 13 => D + C = 13 => C = 13 – D;

C – D = 6 => 13 – D – D = 6 => 2D = 7 => D = 3.5,

C = 13 – D = 13 – 3.5 = 9.5;

A = D = 3.5;

A + B = 8 => B = 8 – A = 8 – 3.5 = 4.5.

ответ: A = 3.5; B = 4.5; C = 9.5; D = 3.5

Проверка.

3.5 + 4.5 = 8, 9.5 – 3.5 = 6,

3.5 + 9.5 = 13, 4.5 + 3.5 = 8.