New answers tagged beginners
You can pronounce the beginning the same like four if you have not been understood the first time. Then you'd just say vier - zehn. But if you pronounce it in one word firzehn. The whole word sounds more balanced to me. Also I have a hard time pronouncing zehn after saying vier like if vier was standalone. There is just not enough air left. So I have to ...
I want to add, as a complement to Martin H.'s answer, where the pronunciation of 15 is evoked, the following map from the Atlas zur deutschen Alltagssprache
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