25*6.4*12.5
=25*0.4*2*0.8*12.5
=10*2*10
=200.
2/3-1/6=4/6-1/6=3/6=1/2.
3/4+1/8= 6/8+1/8=7/8.
6/7+7/9-6/7+7/9=14/9.