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