Use words to describe the relationship between corresponding number of hours and weeks

There are "40 hours per week."     [unit rate means "how many of these for one of those"]

Notice how units cancel when we use the Multiplicative Identity (that is 1 -- multiply any value by 1 and it does not change):

    1  =   (40 hours)/(1 week) = (1 week)/(40 hours)        [choose the value needed to cancel units]

  120 hours

  (120 hours) * 1

  (120 hours) * (1 week)/(40 hours)

  (120/40) weeks                      [hours cancel, leaving only weeks[

     3 weeks

    5 weeks

   (5 weeks) * 1

   (5 weeks ) * (40 hours)/(1 week)

    (5*40) hours                    [note that weeks cancel]

     200 hours