在浓度为20%的糖水中加入30克水,浓度变为15%,再加入多少克糖,糖水的浓度变为25%

(1)20%的糖水x克!
(2)20x% == (x+30)*15%
x==90g
(3)再加入y克糖,糖水的浓度变为25%!
(4)(90*20%+y)/(90+y)==0.25
y == 6g
(5)再加入6克糖,糖水的浓度变为25%!
ok!