X/10-(13-11)=X/15
X/10-2 = X/15
X/10- 20/10 = X/15
3X/30 - 60/30 = 2X/30
(3X - 60)/30 = 2X/30
30(3X - 60) = 2X * 30
90X - 1800 = 60X
30X = 1800
X= 60
这个够详细了吧