alleryTest 91 for 7: remove the last digit 1, then subtract twice it from 9. What is the result?
A5
B6
C7
D8
Answer & Solution
Correct answer: C. 7
1. Split 91 into 9 and the last digit 1.
2. Compute 9 - 2 x 1 = 7.
3. 7 is divisible by 7, so 91 is divisible by 7 (91 = 7 x 13).
_Source: Wikipedia, Divisibility rule, section Divisibility by 7_
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