If you are doing it for 3 2/7 weeks into day then it would be a week is seven days so 2/7 of a week is 2 days. after that we also need to calculate how many days are in three weeks, to do that we multiply 3 by 7 which equals 21. take 21 and add 2 in order to get 23 days in 3 2/7 weeks
