设运来面粉x袋，3.2x+x=8404.2x=840x=840÷4.2解得，x=200所以，运来面粉200袋
设面粉为x袋x+3.2x=840x=200
设面粉x袋x+3.2x=840 4.2x=840 x=200袋大米840-200=640袋
