Can someone explain to me how to do this

rank | 1st | 2nd |TOTAL
|+|
a     | 6   +    9   |  =15
b     | 7   +   3    | =16
c     | 9   +   7    | =23
d     | 6   +   7    | =22
e     | 8   +  1     | =?

If you add the 1st in line a (6) and you add it to the 1st in line b (7) + the 2nd in line b (3) you get 16. Repeat the same logic/pattern to all c,d..
This is the pattern to all this puzzle;

So the before last term is :  9+6+7 = 22
and the last term : 6+8+1 = 15

The first one minimumi hope my answer you.