it's takes 20 minutes to watch the movie previews the preview start at 7 o'clock what time the movie start
The previews begin at 7 o'clock, so you add the twenty minutes left. 7:00+0:20=7:20
They begin at 7:20.
the movie begins at 7:20 because if the previews are 20 mins long the movie would start 20 mins later