The recipe called for one-third of a cup of flour, but i accidentally put in three cups. why does one-third contain a hyphen in this sentence? i need the answer pls

Doulingo is good. you can also watch movies that are in french, i am using this tactic  for japanese  and it a lot. listing to music with  french  lyrics would work well too because  you could absorb  the words while having fun.hope this ! : )