设服装进价为a ,共有b件鸭绒服 ,降低了x
aX(1+30%)X90%b+aX(1+30%)X (1-90%)bx =aX(1+30%)XbX85%
两边同时约去aX(1+30%)Xb
0.9+0.1x =0.85
x =0.5
=50%