（2/69+2/71）*23+25/71
=（2/69)*23+2/71*23+25/71
=2/3+46/71+25/71
=2/3+1
=5/3
6/7-5/9*6/7
= 6/7*(1-5/9)
= 6/7*(4/9)
=8/21