Binomial coefficients, as combinatorial quantities expressing the number of ways of selecting objects out of without replacement, were of interest to ancient Indian mathematicians. The earliest known reference to this combinatorial problem is the "Chandaḥśāstra" by the Indian lyricist Pingala (c. 200 BC), which contains a method for its solution. The commentator Halayudha from the 10th century AD explains this method using what is now known as Pascal's triangle. By the 6th century AD, the Indian mathematicians probably knew how to express this as a quotient formula_6, and a clear statement of this rule can be found in the 12th century text "Lilavati" by Bhaskara.