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
