（2.7-4.25x2/3）+0.5x0.625x0.5=2.7-17/4x2/3+1/2x5/8x1/2=2.7-17/6+5/32=648/240-680/240+37.5/240=（648-680+37.5）÷240=5.5/240=11/480
