HOW TO FIND A, B, C & D

CORRECT ANSWER:

A+B=8
C-D=6
A+C=13
B+D=8

A+B=8 and B+D=8 so A+B=B+D. Therefore A=D
Now, A+C=13 and C-D=6 so 13-A=6+D so 13-D=6+D. Hence 2D=7. So D=3.5 and A=3.5
A+B=8 so 5+B=8 hence, B=4.5
A+C=13 so 3.5+C=13 hence C= 9.5

Our four numbers A,B,C & D are 3.5, 4.5, 9.5 and 3.5 respectively.