3.6×1994.4+2006.9×6.4
=3.6×1994.4+(1994.4+12.5)×6.4
=3.6×1994.4+1994.4×6.4+12.5×6.4
=1994.4×（3.6+6.4）+12.5×8×0.8
=19944+80
=20024