（0.75*42.6-0.574*25）/3=(0.75*42.6-57.4*0.25)/3=[(1-0.25)*42.6-57.4*0.25]/3=[42.6-0.25*42.6-57.4*0.25]/3=[42.6-0.25*(42.6+57.4)]/3=[42.6-25]/3=17.6/3=5.866.