Which two rules work by adding up the digits?
A3 and 9
B5 and 10
C4 and 12
D2 and 6
Answer & Solution
Correct answer: A. 3 and 9
1. Both replace the number with its digit sum.
2. Then the sum is tested against the divisor.
3. They are the rules for 3 and 9.
_Source: OpenStax, Contemporary Mathematics, Ch 3 'Real Number Systems and Number Theory'_
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