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50's |
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'50s |
There's no need for the apostrophe before the S in decade names like 50s and 60s, since there are no omitted letters. The term may be written '50s since "19" is being omitted, but "50s" is fine too. Writers who wish to have their references to decades clearly understood in the 21st century would be well advised not to omit the first two digits.