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When abbreviating "fifties", there are a couple of commonly used formats: '50s and 50s. Both ways are acceptable, but '50s is considered the more formal and conventional way to abbreviate it.
Using an apostrophe before the numbers ('50s) signifies that the numbers are a contraction or abbreviation for a larger word or phrase. In this case, it represents the omission of "19". It is a traditional style used for denoting decades.
On the other hand, writing 50s without the apostrophe is a more modern and simplified form of abbreviation. It is commonly used in informal writing and is often preferred in casual contexts.
So, either '50s or 50s can be used, depending on the level of formality and personal preference.