1982 AMC 12 Problem 26

Attempt Problem 26 of the 1982 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1982 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

26.

If the base 88 representation of a perfect square is ab3c,ab3c, where a≠0,a\ne0, then cc is
00
11
33
44
not uniquely determined
Answer: B
Concepts:modular arithmeticperfect squarenumber basequadratic residue
Small hint:
Only the last two base-88 digits matter modulo 6464
Big hint:
List the quadratic residues modulo 6464 that lie between 30830_8 and 37837_8
Solution:
The last two octal digits 3c83c_8 represent 24+c(mod64).24+c\pmod{64}. Squares modulo 6464 in the range 24,…,3124,\ldots,31 include only 25.25. Thus 24+c=25,24+c=25, so c=1.c=1.
Therefore, the correct answer is B.
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