Delete an app completely (special case) and question regarding empty trash
All the apps installed from the App Store are stored in the /Applications folder.
Go to the Applications folder (Finder menu -&gt; Go -&gt; Applications or check the Finder window sidebar for this folder), check if the app you want to delete is present there. If it isn't, then it's gone. Else, move it to Trash and then empty it.

If it doesn't help, please clarify how are you determining that 20GB is still occupied by the app.