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