Brahmagupta gives the following rules concerning operations carried out on what he calls “fortunes” (dhana), “debts” (rina) and “nothing” (kha).
A debt minus zero is a debt.
A fortune minus zero is a fortune.
Zero (shunya) minus zero is nothing. (kha).
A debt subtracted from zero is a fortune.
So a fortune subtracted from zero is a debt.
The product of zero multiplied by a debt or fortune is zero.
The product of zero multiplied by itself is nothing.
The product or the quotient of two fortunes is one fortune.
The product or the quotient of two debts is one debt.
The product or the quotient of a debt multiplied by a fortune is a debt.
The product or the quotient of a fortune multiplied by a debt is a debt.
Modern algebra was born, and the mathematician had thus formulated the basic rules: by replacing “fortune” and “debt” respectively with “positive number” and “negative number”, we can see that at that time the Indian mathematicians knew the famous “rule of signs” as well as all the fundamental rules of algebra.
(source: The Universal History of Numbers - By Georges Ifrah