"My grandfather is fifty."

Translation: Min bedstefar er halvtreds.

[Daniel98101]

danish people must be wizards at math

[AnCatDubh]

Half-sixty?…

[Xneb]

It's actually short for "halvtredsindtyve" or "Two and a half times twenty", but everyone would say it in the way of Duolingo

[epac-mcl]

It comes from the old "halvtredje gang tyve" = 2½ x 20 = 50.