Is he that bad at math?
Three weeks = 21 days he used 1 day, SO that leaves 20 days.
Maybe if you write it down like that he'll be able to understand what you are saying. ;\)

Hope things are going well with YOU.

JAK

You don't get to choose how you're going to die. Or when, you can only decide how you're going to live now. ~Joan Baez