根据题意知:
X/3=Y/4=Z/5
设X/3=Y/4=Z/5=K
则X=3K,Y=4K,Z=5K
所以
(X+Y)/(X-2Y+3Z)
=(3K+4K)/(3K-2*4K+3*5K)
=7K/(10K)
=7/10