(3/4-3/5）÷7/10
=(15/20-12/20)x10/7
=3/20 x 10/7
=3/14
（2/3+2/9）÷20/21
=(6/9+2/9)x21/20
=8/9 x 21/20
=2/3 x 7/5
=14/15
1-5/14÷5/7
=1-5/14 x 7/5
=1-1/2
=1/2
1/4÷（2/3+1/2）×4/5
=1/4÷7/6 x 4/5
=6/28 x 4/5
=6/35
1) =(3/20)*(10/7)=3/14
2) =(6/9-2/9)*(21/20)=(4/9)*(21/20)=7/15
3) =1-(5/14)*(7/5)=1-1/2=1/2
4) =(1/4)*(4/5)/(7/6)=(1/5)*(6/7)=6/35
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