2023 AMC 12B Problem 18

Attempt Problem 18 of the 2023 AMC 12B 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 2023 AMC 12B solutions, or check the answer key.

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18.

Last academic year Yolanda and Zelda took different courses that did not necessarily administer the same number of quizzes during each of the two semesters. Yolanda's average on all the quizzes she took during the first semester was 33 points higher than Zelda's average on all the quizzes she took during the first semester. Yolanda's average on all the quizzes she took during the second semester was 1818 points higher than her average for the first semester and was again 33 points higher than Zelda's average on all the quizzes Zelda took during her second semester. Which one of the following statements cannot possibly be true?

Yolanda's quiz average for the academic year was 2222 points higher than Zelda's.

Zelda's quiz average for the academic year was higher than Yolanda's.

Yolanda's quiz average for the academic year was 33 points higher than Zelda's.

Zelda's quiz average for the academic year equaled Yolanda's.

If Zelda had scored 33 points higher on each quiz she took, then she would have had the same average for the academic year as Yolanda.

Answer: A
Concepts:weighted meaninequalitylogical deduction
Difficulty rating: 1800
Solution:

Subtracting Zelda's first-semester average from all four semester averages does not affect the comparison. We may therefore write the semester averages as 0,180,18 for Zelda and 3,213,21 for Yolanda. Let pp and qq be the fractions of Yolanda's and Zelda's quizzes, respectively, that occurred in the second semester. Their yearly-average difference is (3+18p)18q=3+18(pq). (3+18p)-18q=3+18(p-q). Because 0<p,q<1,0\lt p,q\lt 1, this difference is less than 21,21, so it cannot be 22.22.

The other options really can occur. Taking (p,q)=(14,34)(p,q)=(\tfrac14,\tfrac34) makes Zelda's average higher; taking p=qp=q makes Yolanda's average 33 points higher (and also verifies the last option after adding 33 to every Zelda score); and taking (p,q)=(13,12)(p,q)=(\tfrac13,\tfrac12) makes the yearly averages equal. Each displayed fraction can be realized by positive integer quiz counts.

Thus, the correct answer is A.

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