设杨树有X棵,则杉树有(91-X)棵.
1/4X=(91-X)2/5
1/4X=91*2/5-2/5X
(1/4+2/5)X=91*2/5
13/20X=182/5
X=182/5*20/13
X=56
91-X=91-56=35
答:杨树有56棵,杉树有35棵.
设杨树有X棵,则杉树有(91-X)棵.
1/4X=(91-X)2/5
1/4X=91*2/5-2/5X
(1/4+2/5)X=91*2/5
13/20X=182/5
X=182/5*20/13
X=56
91-X=91-56=35
答:杨树有56棵,杉树有35棵.