7.9×0.35+2.1×0.35=0.35×(7.9+2.1)=0.35×10=3.5
由原题得（7.9+2.1）x0.35=10x0.35=3.5
7.9x0.35+2.1x0.35=0.35×(7.9+2.1)=0.35×10=3.5
