"How far away is the beach and the sea?"
Translation:Pa mor bell i ffwrdd yw'r traeth a'r môr?
Although less formal I'm fairly sure "Faint mor bell yw'r traeth a'r môr?" is correct
That sentence seems to have been deleted - check that you are using the latest version of the app if that is how you are using Duo.
The English should be 'are' rather than 'is'.
Pa mor bell ...? is the usual pattern taught on 'Welsh for Adults' courses. Faint mor bell..? also occurs in the wild.