Meaning of the Term

    What does the term’ Casting out the nines’ refers to?

Answer:   Checking Addition and Multiplication: check the accuracy of additions and multiplications was introduced about a thousand years ago by the Arabs.

                To check product of a multiplication nines are’ Cast out’ of each factor in the multiplication equation, the remainders are then multiplied and nines are ‘Cast out’ again. The remainders, if they are unequal at this stage, the multiplication is incorrect.

                 However, it does not follow that if the remainders are equal the multiplication is correct. But it shows that the chances are that it is correct. So, the process of ‘Casting out nines’ has only a limited application.

                ‘Casting out nines’ can be used to check for mistakes in the addition of numbers also in the same manner.