The arrangement law of 1. interval phenomena. Tree planting phenomenon: (1) both ends are planted, interval number+1= plant number (2) both ends are not planted, interval number-1= plant number (3) If one end is planted and the other end is not planted, interval number = plant number. In the end-to-end closed arrangement, the number of objects is equal to the number of intervals. Similar phenomena include sawing wood and climbing stairs. There is an 800-meter-long expressway. How many seedlings does it take to plant a poplar tree every 20 meters from beginning to end on one side of the expressway? 2. Laws in simple collocation and arrangement. For example, there are three roads from Xiaoming's home to Children's Palace and four roads from Children's Palace to Xinhua Bookstore. How many walking routes are there from Xiaoming's home to Xinhua Bookstore? 3. An example of the law in the simple cycle phenomenon: When Xiaohong practiced hard-pen calligraphy at home, she wrote "Beijing Olympics, Beijing Olympics-"in turn, so what should the 24th word be?