Today is Amy\textquotesingle s birthday! She takes photos with her four close friends. All of them stand in a line and they make Amy stand in the middle. There are~\uline{~~~~~~~~~~}~different ways for them to form the line. 
Overseas Competition->Knowledge Point->Counting Modules->Law of Addition and Multiplication->Principle of Multiplication
$$4 \times 3 \times 1 \times 2 \times 1 = 24$$. 