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A number of the form 5^n - 1 is always a multiple of which greatest number?
A2
B8
C4
D5
Answer & Solution
Correct answer: C. 4
5^1 - 1 = 4 and 5^2 - 1 = 24, so 4 is the greatest common divisor, and 5^(k+1) - 1 = 5(5^k - 1) + 4 carries it forward.
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