Where is 1 fifth on a ruler?

Look at the markings between zero inches and one inch. There should be a larger mark right in the middle between the two. That's 1/2 (one half) inch. Right in the middle between 0 and 1/2 is a smaller mark - that's the 1/4 (one quarter) inch mark. On the other hand, the mark halfway between 1/2 and 1 inch is the 3/4 (three quarter) inch mark. In other Words, the distance from zero to one inch can be divided into four equal steps with three marks in between; those are, respectively, 1/4 inch, 2/4 inch (= 1/2 inch), and 3/4 inch. 4/4 inch is equal to 1 inch.