84.8+27.5-(34.8+7.5)=84.8+27.5-34.8-7.5=(84.8-34.8)+(27.5-7.5)=50+20=70
原式=5/27×3/8+5/27×5/8 =5/27×(3/8+5/8) =5/27×1 =5/27
