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probname); ?>
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Supplies: nid, "Supplies"); ?> Pedagogy: nid, "Pedagogy"); ?>

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Solution: nid, "Solution"); ?>
Problem

Write an integer $a$ in terms of its decimal expansion: $a = a_0 + 10 a_1 + 100 a_2 + \cdots + a_d 10^d$. Prove that $a$ is divisible by 7 if and only if $(-2)^d a_0 + (-2)^{ d-1 } a_1 + (-2)^{ d-2 } a_2 + \dots + a_d$ is.

Details
Source Title: Apoorva Khare, a sixth form student in Orissa, India, see Electronic Journal of Undergraduate Mathematics {3} (1997), 1--5.
Authors
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References
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Problem Sets This Problem Belongs to:
parent_nid; ?> Set:
VARIABLES
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DEFINITIONS
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