4分之1÷5分之1-4分之3
=5/4-3/4
=1/2
7分之5÷{（1减4分之3）÷20分之3}
=5/7÷{1/4÷3/20}
=5/7÷5/3
=3/7要简便运算只能这样，没法简便啊