由X:Y=4:7 X:Z=3:5 X-2Y+3Z=30得 X=4/7Y Z=5/3Y 将X=4/7Y ,Z=5/3Ｙ代入 X-2Y+3Z=30得 4/7Y-2Y+3×5/３Y＝３０,解得Y=8.4 将Y=8.4代入X=4/7Y ,Z=5/3Y 解得X=４．８,Ｚ＝１４