3x+7y+z=6.3 ...①
4x+10y+z=8.4 ...②
② - ①得x+3y= 2.1,即x= 2.1-3y .③
将③带入①得6.3-9y+7y+z=6.3,即z=2y
x+y+z= 2.1-3y+y+2y=2.1