题目内容：

Xiao Hong read a story book for three days.On the first day,he read 25 pages.Onthe second day ,he
read2/5 of the book.on the third day,he read9/20 of the book.then there are ()pages in the book.

优质解答

第一天读了25页,第二天读了余下的2/5,第三天读了总页数的9/20.一共读了三天.
设这本书一共有x页.
则25+（2/5）（x-25)+（9/20)x=x
解得x=100页