Ancient India
Decimal Number System
Although many civilizations used a base ten system to count objects, the symbols and expressions used were incredibly diverse. With the introduction of the Hindu-Arabic numeral system, many civilizations began to adopt it when solving equations because representing numbers as a string of digits (including decimals) was pretty simple. More significantly, the Hindu-Arabic numeral system was the foundation for the symbols we use in our base ten numeral system today.
Brahamagupta
Brahmagupta is most famous for obtaining a formula for the linear diophantine equations in the form of ax + by = c. In addition, he also obtained a form of the modern-day quadratic formula and even studied Pell's equation (a mathematician from the 17th century who came up with the diophantine equation x^2 - ny^2 = 1
Bhaskara II
In addition to solving algebraic equations, Bhaskara II distinguished between indeterminate and determinate equations. Indeterminate equations have more than one solution, unlike determinate equations, which only have one solution. Like Brahamgupta, he studied diophantine equations and even solved Pell's equation. Moreover, Bhaskara II wrote two books (Vija - Ganita and Lilvati), which covered his work in Sanskrit, effectively encouraging more Indians to become mathematicians.