0.75×15+4.5×2.5
=(1-0.25)*15+45*0.25
=15-15*0.25+45*0.25
=15+30*0.25
=15+7.5
=22.5
3.6×31.4+43.9×6.4
=3.6*31.4+(31.4+12.5)*6.4
=31.4*(3.6+6.4)+12.5*6.4
=31.4*10+(10+2.5)*6.4
=314+64+2.5*6.4
=378+2.5*(6+0.4)
=378+2.5*0.4+2.5*6
=378+1+15
=394