a when there is no preposition at all in English

Liguei-te duas vezes a* semana passada e tu não me ligaste.
I called you two times last week yesterday but you didn't call back.
Tinham de ser trocadas três vezes ao dia.
They have to be changed three times a day.

* There is no preposition in Portuguese either. The a is an article. However in English there is not even an article.

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