7.8X3.4-15X0.78+78X0.81=7.8X(3.4+1.5+8.1)=7.8X13=8X13-0.2X13=104-2.6=101.4
7.8*（3.4-1.5+8.1）=7.8*10=78
7.8×3.4-15×0.78+78×0.81=78×0.34-0.15×78+78×0.81=78×（0.34-0.15+0.81）=78×1=78
