3/5×（2/3＋5/4）
=3/5x2/3+3/5x5/4
=2/5+3/4
=23/20
4/5÷7/4×35/8
=16/35x35/8
=2
12/7÷（2/3-1/4)
=12/7÷5/12
=144/35