15:(312/17+25/17)+(4,2-23/5):4
15:(312/17+25/17)+(4,2-23/5):4= 15/1*17/337+(21/5-23/5):4= 255/337-2/5*1/4= 255/337-1/10=2550/3370-337/3370=2213/3370