§Puzzle terminology · math
Digit sum
The sum of the individual digits of a number. Used as a hint in NumGrid and as a classic divisibility test.
Origin and context
A number is divisible by 3 if its digit sum is divisible by 3; divisible by 9 if its digit sum is divisible by 9. These tests work because powers of 10 are congruent to 1 mod 3 and mod 9.
Related terms
Frequently asked
What is Digit sum?
The sum of the individual digits of a number. Used as a hint in NumGrid and as a classic divisibility test.
Where does Digit sum come from?
A number is divisible by 3 if its digit sum is divisible by 3; divisible by 9 if its digit sum is divisible by 9. These tests work because powers of 10 are congruent to 1 mod 3 and mod 9.
What's related to Digit sum?
Related puzzle terms include NumGrid, Parity. See the linked entries for full definitions.