The mint mark on Jefferson nickels has been in 3 different places over the years. Post-1980, mint marks should be near the date. If there is no letter there you'll need to have the coin examined in person. AFAIK there have been no reports of 1988 nickels being struck without a mint mark.
a quarter
Just to make sure take it to a dealer or collector for and opinion, but it's likely from a filled die that caused the mint mark not to strike up.
This date regardless of mint mark is valued for the silver and is worth about $3.25
It's worth 25 cents.
If you got in change spend it, in 1968 the only mint marks used were D & S a coin without a mint mark was struck in Philadelphia.
No mint mark means it was minted in Philadelphia. The quarter is worth exactly 25 cents.
It is worth .25¢ The absence of a mint mark simply means it was minted at Philadelphia.
All the Washington Quarters minted in the year 1989 carried a mint mark: "P," "S," or "D." The P and D minted coins are worth in MS60 condition: 75 cents; the S coin was minted only in proof and its value is: $3.00.
Not enough info what condition is it in and what mint mark
25 cents.
The value of a 1936 quarter depends on its condition and mint mark. Generally, a circulated 1936 Washington quarter is worth around $4 to $10, while uncirculated examples or those with special mint marks (like the "D" for Denver or "S" for San Francisco) can be worth more. To get an accurate valuation, it's best to consult a coin dealer or use a coin grading guide.
No U.S. coins dated 1966 have a mint mark. The '66 quarter is worth 25 cents.