数学请教

xp635

+	X	Y	Z
4	1	-5	m
e	7	n	10
f	2	-4	5

the value of m+n?

请各位牛人解释这题是怎么做滴阿？ 不甚感激

xp635

I got a interpretation like follow:

X+4=1  so X=-3

X+e=7  so e=10

X+f=2  so f=5

second column

Y+4=-5  so Y=-9

Y+e=n >>-9+10=1 >> n=1

third column

Z+e=10>>Z+10=10>> Z=0

Z+4=m>>0+4=4 >> m=4

in conlusion: m+n=4+1=5

your comments are appreciated

xp635

this is from GMATPrep, anybody shows some direction?