①9/10-（2/5+1/4）=9/10-2/5-1/4=9/10-4/10-1/4=5/10-1/4=1/2-1/4=1/4
②3-6/13÷9/26-2/3=3-6/13×26/9-2/3=3-4/3-2/3=3-2=1
③1.05×（4.8-0.8）÷2.1=1.05×3÷2.1=1.05×(3÷2.1)=1.05×1/0.7=1.05÷0.7=1.5
④（7/8-1/4）×（2/3-1/9）=（7/8-2/8）×（6/9-1/9)=5/8×5/9=25/72