.1又2/7+7/9-(2/7-2/9)
=1又2/7+7/9-2/7+2/9
=1又2/7-2/7+7/9+2/9
=1+1
=2
2.9又1/4*7/8÷(9又1/4÷8/7)
=9又1/4*7/8÷9又1/4*8/7
=9又1/4÷9又1/4*7/8*8/7
=1*1
=1
解设原来有5X、4X
(5X-4X):4X=3:5
X=20
4*20=80
.1又2/7+7/9-(2/7-2/9)
=1又2/7+7/9-2/7+2/9
=1又2/7-2/7+7/9+2/9
=1+1
=2
2.9又1/4*7/8÷(9又1/4÷8/7)
=9又1/4*7/8÷9又1/4*8/7
=9又1/4÷9又1/4*7/8*8/7
=1*1
=1
解设原来有5X、4X
(5X-4X):4X=3:5
X=20
4*20=80