ABSTRACT
Let us begin with a simple motivating problem: If today is Monday, what day will it be 200 days from now? At first glance this seems like a cumbersome problem to answer, until we recognize a simple trick: 7 days from now, it will be Monday again. It will also be Monday 14 days from now, 21 days from now, etc. In particular, it will be Monday 196 days from now, because 196 is a multiple of 7. So, since we “reset the clock” at day 196, the original question posed is equivalent to asking what day it will be 4 days from now, and the answer to that is obvious: Friday.
