The number on each corner is equal to the average of the middle numbers on its non-adjacent sides.
Therefore,
c=6*2-7=5
b=2*2-5=- 1
From b+6=c+f
f=6- 1-5=0
Then e=0-(- 1)= 1.
a=(7+ 1)/2=4
d=a+6-7=3
To sum up, the formula can be obtained by substitution.
(a b+ CD+ef)/(a+b+c+d+e+f)=(-4+ 15+0)/(4- 1+5+3+ 1+0)= 1 1/ 12