I think the fact that Duo allows some typos without marking the answer wrong causes issues when using digits, because the sentence is just one keystroke off from the correct answer.

Uh, hi?

Do you mean in English? I think it's a contraction of "# of the clock."

Take your upvote and get out.

A historically developed contraction of "of (the) clock". Cf "cat-o-nine-tails" = "cat of nine tails".