(78.6-0.786×2.5＋0.75×21.4)÷15×1997=（393/5-393/500*5/2+3/4*107/5）/15*1997=（393/5-393/200+321/20）/15*1997=（393/5+2817/200）/15*1997=18537/200/15*1997=12339.463