The 3rd sentence: The decor(with an apostrophe on top of the e) of the mens' room looked like it had not been changed since the 1970's.
The reason why because 'men's' doesn't need an apostrophe at the end because it's a singular plural and the men do not belong to room, much.
However, decor(with an apostrophe on top on the e) needs to have that because it comes from french, in addition 1970's needs an apostrophe because it's a year and it's a proper noun and you're adding an 's at the end.
Hope this helped. :
