There are $3$ entrances to an amusement park. Len, Joe, Angle, and Jack are going to enter the park. They can choose the same entrance, or different ones. Each entrance must be entered by at least one person. The order in which the four people enter the park is uncertain. How many different ways are there for them to enter the park? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations