"I'm going to Swansea at ten to three."
Translation:Dw i'n mynd i Abertawe am ddeg munud i dri.
The BBC Welsh grammar ( http://www.bbc.co.uk/wales/learnwelsh/pdf/welshgrammar_allrules.pdf ), on page 65, section E (Numbers), point 8.a, says:
« 'deg' (ten) becomes ‘deng’ before ‘m’ »
(among the examples shown is deuddeng munud).
Also Wikipedia, in the 'Welsh numerals' page ( https://en.wikipedia.org/wiki/Welsh_numerals ), states:
« The words for "ten", "twelve", and "fifteen" (deg, deuddeg, pymtheg) have the alternative forms deng, deuddeng, pymtheng used before nasals. »
So how come Duolingo considers am ddeng munud i dri wrong?
'am ddeng munud i dri' is one of the accepted alternatives here. 'ddeng' is a bit archaic and is not taught on the standard courses for adults.
Thank you for the explanation.
From now on I'll avoid using deng / deuddeng / pymtheng in the exercises.