7/10x48+48x3/10=(7x48)/10+(48x3)/10=48x(7+3)/10=48;24x(5/12-1/24)=24x(10/24-1/24)=24x9/24=9; (20+4/5)除4/5=（100/5+4/5)x(5/4)=(104/5)x(5/4)=104/4=26;12x(1/4+1/6-1/3)=12x(3/12+2/12-4/12)=12x1/12=1 ...