Early Quenya
nelkestya
adjective. one thirteenth
nelkeast(y)a
adjective. one thirteenth
nelkesto
fraction. one thirteenth
nelkeasto
fraction. one thirteenth
nelkeatto
fraction. one thirteenth
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A word appearing in the Early Qenya Grammar of the 1920s as ᴱQ. nelkeatto or nelkesto “[one] 13th” with adjective forms ᴱQ. nelkestya or nelkeast(y)a (PE14/51, 84), combinations or ᴱQ. nelkea “thirteen” with the 1920s fractional suffix ᴱQ. -sto.
Neo-Quenya: Since I prefer nelquë “thirteen” from Tolkien’s later writings, I would update this form to ᴺQ. nelquesta “one thirteenth” for purposes of Neo-Quenya.