It is worth face value unless it is still in the original mint package.
25 cents.
The date is 1776-1976 and it's only a quarter.
Yes. There were US quarter dollars struck in 1975 but all of them carried the date 1776-1976. Likewise, all 1976 quarter dollars also carried the 1776-1976 date. There is no way to identify the coins struck in 1975 from those struck in 1976.
US$666.66
Only face value unless in mint packaging.
All US Quarters made in 1975 & 1976 have the dual date 1776-1976 so if the coin has a date of 1975 it's not a US coin.
All bicentennial coins were minted in the years 1975 and '76.
Not a lot, and most people ask too much.
In 1976, the US national average price of a gallon of regular unleaded gasoline was 61 cents - equivalent to about $2.34 per gallon in 2010.
I remember paying 1.39 in that time.
Please check your quarter again and post a new, separate question. The first US quarters were minted in 1796. If you have a dual-dated 1776-1976 quarter it's one of the billions minted for the US Bicentennial, and is worth only face value.
In the US in 1976, the average price of gasoline was 57 cents per gallon. This equates to about $2.35 per gallon in 2014 dollars.