The divisibility test that an integer is divisible by 9 iff the sum of its digits is divisible by 9.
Rule of Nines
See also
Casting Out Nines, Divisibility TestsExplore with Wolfram|Alpha
References
Flannery, S. and Flannery, D. In Code: A Mathematical Journey. London: Profile Books, pp. 115-116, 2000.Referenced on Wolfram|Alpha
Rule of NinesCite this as:
Weisstein, Eric W. "Rule of Nines." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/RuleofNines.html