SOLUTION: Suppose today is Tuesday. What day of the week will it be 100 days from now?

Divide 100 by 7
   14
7)100
   7
   30
   28
    2

The quotient is the number of full weeks and the remainder is the number
of days after the largest whole number of weeks 100 days contains.
Since 100 days is 14 weeks and 2 days, it will be two days after a 
Tuesday or Thursday.

Edwin