Nina and four of her friends are lining up to take a picture. If Nina has to be in the middle, how many different ways can the five friends line up? 
Overseas Competition->Knowledge Point->Counting Modules->Law of Addition and Multiplication->Queuing Problems
Since Nina has to be in the middle, then there are four friends who can line up in different ways. We can write the equation as $$4\times3\times2\times1=24$$. 