In Genesis 10:20&30 it is written according to their languages and in Genesis 11:1 says the whole earth had one language and one speech. How is that possible?

Thank you for the question. Genesis 10 and 11 are not in the chronological order(meaning its not in the order in which the events took place). The writer of genesis has given the details of Noah's descendants and where they settled in the chapter 10. Chapter 11 gives why people started settling in different places and how the different languages came into being. The events in chapter 11 is not happening after the events in chapter 10, but happens sometime after the flood. Just that it is explained in detail in chapter 11. Attaching a link with the detailed answer... Hope it helps and your question is answered.