答:8.4×0.25+0.75=2.1×4×0.25+0.75=2.1×(4×0.25)+0.75=2.1×1+0.75=2.1+0.75=2.85
＝8.4*0.25+0.25*3＝0.25*（8.4+3）＝0.25*11.4＝2.85
