Very much depends on whether you need to check in luggage. My guess is that bag drop is closed 20 minutes prior to departure, but I might be wrong. Picking up luggage can easily add 15-30 minutes as well. Even on relatively small airports like Copenhagen, some gates are at least 10 min of brisk walking from the security, so 20 minutes is really pushing it without prior familiarity with the airport. But yes, it can be done.