@vi Maybe someone told you already but some French speaker (Belgian and Swiss mostly) will say :
70 - septante
80 - huitante / octante
90 - nonante

Most French people will understand you, after some thinking (it's not natural for someone not used to it, but it's understandable).

But that's not in the "norm", not "allowed" by the "Académie française", so I don't think Duolinguo will allow it. ^^