20/7=2...6, if today is Sunday, it will be Saturday in 20 days. Because there are 7 days in a week, divide 20 by 7 to get the integer 2, which proves that these 20 days are composed of two weeks, and the remainder 6 is less than a week (7 days) and one day less than a week, so the day before Sunday, that is, 20 days later, is Saturday.