"My teacher is a bit scary."
The following was posted by a native Japanese on HiNative.
Just to note, the two can be used simultaneously when quantity or time is concerned, otherwise you need to be a little discrete.
I can't see the date on mobile but in the past Japanese courses on Duo didn't have Kanji, furigana is even more recent addition. So you'll find quite a lot of comments around where people looked up the kanji or the furigana and typed it here so that others don't have to. In other words, it probably made sense to do this back then even though it's superfluous now.