Difference between revisions of "1968 AHSME Problems/Problem 33"

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== See also ==
 
== See also ==
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[[Category: Intermediate Algebra Problems]]
 
[[Category: Intermediate Algebra Problems]]
 
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{{MAA Notice}}

Latest revision as of 01:53, 16 August 2023

Problem

A number $N$ has three digits when expressed in base $7$. When $N$ is expressed in base $9$ the digits are reversed. Then the middle digit is:

$\text{(A) } 0\quad \text{(B) } 1\quad \text{(C) } 3\quad \text{(D) } 4\quad \text{(E) } 5$

Solution

Call the number $\overline{abc}$ in base 7.

Then, $49a+7b+c=81c+9b+a$. (Breaking down the number in base-form).

After combining like terms and moving the variables around, $48a=2b+80c$,$b=40c-24a=8(5c-2a)$. This shows that $b$ is a multiple of 8 (we only have to find the middle digit under one of the bases). Thus, $b=0$ (since 8>6, the largest digit in base 7).

Select $\fbox{A}$ as our answer.

~hastapasta

See also

1968 AHSC (ProblemsAnswer KeyResources)
Preceded by
Problem 32
Followed by
Problem 34
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All AHSME Problems and Solutions

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