3.6*31又2/5+43.9*6又2/5
=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