duration of movie

I am trying to create a short movie. The video is only 20 seconds long and that will be the entire length of the movie. For audio I wanted to add an audio file that is 3 minutes long. When I add the audio it makes the length of the movie 3 minutes. When I edit the audio file to 20 seconds to match the video the length of the movie stays at 3 minutes. How do I shorten the length of the entire movie to 20 seconds? Thanks.