"Sechs und sieben ergibt dreizehn."
Übersetzung:Six and seven make thirteen.
Ich kann auf Deutsch nicht erklaeren. Es tut mir Leid fuer das Englisch:
This is tricky, but it seems to happen in German, too. When two separate things are working together to form one subject, even if they are separated by and, we still use the third person singular conjugation. Since six and seven make thirteen together, they act as one, singular subject and take singular verbs. Odd, but true. And you can see in German sentence, too.