Well, a fifth is one fifth of one, that's why it's called a fifth, it's divided into five. If your question was 'what is a fifth a tenth of?', then the answer would be two. What is a fifth a seventh...
A "fifth" of liquor was 1/5 of a U.S. gallon (128 ounces) or 4/5 of a U.S. quart (32 ounces). So 128/5=25.6 ounces or (4/5)*32=25.6 ounces. The current bottles are 750 ml or 25.36 ounces.