"The temperatures fell last night."
Translation:Temperaturile au scăzut azi-noapte.
Both do indeed mean "last night". I am not sure why that is, but since most nights extend on two different days (except at the poles), one may argue that you can call last night both ieri-noapte and azi-noapte.
However, azi-noapte is more common than ieri-noapte, so you can just stick with the former or you can say noaptea trecută (literally "last night").
you are correct BUT in english ( or in french etc. LAST night does not mean YESTERDAY night. DUO is always asking us to translate word-for word ( literally) why not here ? I perfectly understood that we should give both answers as correct but I did not do it. on purpose.I wanted to see DUO's reaction. Now i have seen it !