The correct preposition here is "between," as it is used when referring to two people or things. The sentence talks about "Hari" and "Prakash," who are two people, so "between" is the most suitable choice.
Thus, the correct preposition is "between," making the sentence: "I sat down between Hari and Prakash."
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?