100*7.9+184*2.1+84*2.9=100*7.9+(100+84)*2.1+84*2.9
=100*7.9+100*2.1+84*2.1+84*2.9
=100*(7.9+2.1)+84*(2.1+2.9)
=100*10+84*5
=1000+420
=1420