History of Hindu Mathematics: A Source Book v1.c8.s10.7

v1.c8.s10.7

Therefore, $n S₁ (mod. 9),$ Similarly $S₁ S₂ (mod. 9),$ $S₂ S₃ (mod. 9),$ $$ $$ $S_k-1 S_k (mod. 9),$ where S_k is a number of one digit only, say n', which is certainly less than or equal to 9.