3.6×31.4+43.9×6.4，
=3.6×31.4+（31.4+12.5）×6.4，
=3.6×31.4+31.4×6.4+12.5×6.4，
=31.4×（3.6+6.4）+12.5×8×0.8，
=31.4×10+100×0.8，
=314+80，
=394．