Translation:A minute is less than eighty seconds.
In this case, fewer is not correct because we know the number (80 seconds). Fewer would only be correct if the sentence was something like "There are fewer seconds in a minute than an hour."
I am a native English speaker from the UK and I did not know this little fact. In fact, I would say that although I generally have a good command of the English language, I have learned a lot more about my native language since I started learning portuguese. This is especially true when considering sentence structure. I have definitely just been taking it all for granted for the past 35 years.
I would argue fewer is correct by that logic...when things are "countable" (like 80) you use fewer. When it's not (like..."less time") you use less.
Can the English not be: "There are less than eighty seconds in a minute"? Duolingo uses similar constructions elsewhere as an option.