Easiest logical question

For easy notation, $a, b, c$ and $d$ will denote the number in the green, orange, grey and yellow boxes respectively

From the picture, we can conclude that $a+b=8, a+c=13, c-d=6$ and $b+d=8$

From $a+b=8=b+d,$ we get $a=d$ by taking $b$ away from both sides

Therefore $c+d=13$ (by replacing $a$ with $d$ )

So we must find a pair of $c$ and $d$ that suffers the constraints $c+d=13$ and $c-d=6$

$13/2=6.5$ , so if the difference was $0$ , then $c$ would be $6.5$

But the difference is $6$ , so $c$ and $d$ should be bigger and smaller by $3$ , respectively

$6.5+3=\boxed{9.5}$

easiest question,answer link