Request of me twenty, receive of me twenty.
Request of me sixty, your behest will be fulfilled.
Request of me eighty, you'll be given the same;
but ask for one hundred, and you will get only sixty.
What am I?
20 minutes is twenty minutes, 60 minutes is sixty minutes, 80 minutes is eighty minutes (or 1:20 hours/minutes), but 1:00 is one hour or only sixty minutes.
numbers entered into a microwave (that means seconds/minutes instead of minutes/hours, but the idea is the same).