The seventh grade first volume P86 page mathematics "all-effect learning" last question.
Solution: Suppose that the standard water consumption per household in a city is X cubic meters.
1.2X+3(9-X)= 16.2
1.2X+27-3X= 16.2
3X- 1.2X=27- 16.2
1.8X= 10.8
X=6
A: The standard water consumption per household in the city is 6 cubic meters.