Cut a piece of rectangular paper with a length of 20cm and a width of 15cm into several square pieces of paper of the same size, and no pieces of paper can be left. What is the longest side length of these square pieces of paper?
把一张长20cm，宽15cm的长方形纸，剪成若干个同样大小的正方形纸片，纸片不能剩余．这些正方形纸片的边长最大是多少cm？
<result>5</result>