If there are more than one book, it is 15 16 3 1 not moved. Xiaoming Mobile 18, Xiaojie Mobile 19. If there is only one book, because the title does not stipulate that a book can only be moved by one person, it can be assumed that Xiaoming moved X books and Xiaojie moved Y books. Then X=2Y, and (15+16+19+20+31)-x-2y =15 or/kloc-. Cancel the answer, and the solution X Y obtained by that answer is an integer, so the book on the right of the second formula equal sign corresponding to this solution is an untouched book. I can't give an answer because it's not convenient for me to calculate. But it gives a concrete idea, just do the math yourself! Sorry, ha! Choosing me is the best. Oh, thank you.