Why is the singular of “week” used in “two-week business trip”? [duplicate]
Solution 1:
I believe these are known as noun adjuncts (Wikipedia). It's a grey area and unfortunately there are no hard and fast rules. I would say that it's a two-week business trip because one would have originally said two-week long business trip (Edit: which on reflection, is just the same... Never mind)
However something may take place "in two weeks' time" or "in one year's time". See the extract from Fowler on the Wikipedia page.