Leah named her son Yehuda (Judah) based on the Hebrew word for “thankfulness”. Rashi said that she was thankful because she had more than her expected fraction of Yaakov’s sons [Rashi on Bereishis 29:35]. Yehuda was Leah’s fourth child. What fraction of Yaakov’s sons did Leah have after giving birth to Yehuda?
4/12 = 1/3rd