How did you differentiate between 57,000 and 50,007? It seems like both would be elfu hamsini na saba.
To avoid confusion, people will say hamsini na saba elfu for 57,000 and elfu hamsini na saba for 50,007. I'm not sure if this is accepted by Duolingo.
Elfu hamsini saba means 57000
Elfu hamsini na saba means 50007
hamsini elfu na saba is 50,0007. The 'na' is always there before the ones digit