3/4-（2/5+3/10）=
=3/4-(4/10+3/10)
=3/4-4/10
=15/20-8/20
=7/20
5/7+1/4—9/4
=5/7-8/4
=5/7-2
=5/7-14/7
=-9/7
3/4+3/6-5/8
=18/24+12/24-15/24
=15/24
5/9+（3/4-1/2）
=5/9+3/4-2/4
=5/9+1/4
=20/36+9/36
=29/36
1/10+3/8-1/2=
=1/10+3/8-4/8
=1/10-1/8
=4/40-5/40
=-1/40
3/5-（2/15）
=3/5-2/15
=9/15-2/15
=7/15