I came across this one on my 1st GMATPrep test as the 3rd question in the test and I wasted almost 3 mins trying to figure out the damn question but finally I did and the key here is that the row+column=the number that is why it's called a addition table 8)

so 4+x = 1
y+4 = -5
z+4=m

now that you know how to approach this question I am sure you will be able to figure out the answer.

Doh! Sadly I knew that but had to do it another time very slowly to not get screwed up...frustrating!!

can anyone show the easiest way to solve it?
Thanks in advance@@

Here is the answer:

you have to find m+n

let find out m and n from the table

m=z+4
n=y+e

m+n = 4+z+y+e

from the table we know z+e=10

so m+n=4+10+y = 14+y

from the table y = -9 (as y+4=-5)

so m+n=5