1966 AMC 12 Problem 26

Attempt Problem 26 of the 1966 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 1966 AMC 12 solutions, or check the answer key.

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

26.

Let mm be a positive integer and let the lines 13x+11y=70013x+11y=700 and y=mx−1y=mx-1 intersect in a point whose coordinates are integers. Then mm can be:
44 only
55 only
66 only
77 only
one of the integers 4,4, 5,5, 6,6, 77 and one other positive integer
Answer: C
Concepts:Diophantine Equationdivisibilitylinear equation
Small hint:
Substitute y=mx−1y=mx-1 into the first line
Big hint:
For integer x,x, the number 13+11m13+11m must divide 711711
Solution:
Substitution gives (13+11m)x=711. (13+11m)x=711. Thus 13+11m13+11m must be a positive divisor of 711=32⋅79.711=3^2\cdot79. Among divisors greater than 13,13, only 7979 is congruent to 13(mod11).13\pmod{11}. Hence 13+11m=79,13+11m=79, so m=6.m=6.
Therefore, the correct answer is C.
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