若3x+7y+z=315,4x+10y+z=420,则x+y+z=________________.

来源：学生作业帮编辑：大师作文网作业帮分类：数学作业时间：2024/11/18 02:02:22

3X+7Y+Z=315 (1)
4X+10Y+Z=420 (2)
(2)-(1):X+3Y=105,所以:X=105-3Y.(3)
把(3)代入(1)中:
3(105-3Y)+7Y+Z=315,化简:-2Y+Z=0,则:
X+Y+Z=105-3Y+Y+Z=105-2Y+Z=105

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