x-( 1/ 13)x+207-x-7 =(207-x-7)* 2
X= 104 (Ben)
So the science and technology book is 207- 104= 103 (this book).
Thinking: x-(113) x is the story book remaining after lending, and 207-x-7 is the popular science book remaining after lending. Because the two books are equal after lending, the right side of the equation is (207-x-7)*2.