This is the 146th most frequent Hungarian word.
százaléka
"Százaléka" means "percentage of it" in English.
Here, 'százaléka' is used to express a specific percentage fraction belonging to the plural noun 'diákok' (students).
A diákok kilencven százaléka befejezte a házi feladatot.
Ninety percent of the students completed the homework.
In this sentence, 'százaléka' specifies the fractional part of 'vízmolekulák' (water molecules) that consists of oxygen.
A vízmolekulák egy százaléka oxigénből áll.
One percent of water molecules is composed of oxygen.
Here, 'százaléka' refers to the proportional part of 'bevétel' (revenue) being allocated to marketing expenses.