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 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 points higher than her average for the first semester and was again 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 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 points higher than Zelda's.
Zelda's quiz average for the academic year equaled Yolanda's.
If Zelda had scored points higher on each quiz she took, then she would have had the same average for the academic year as Yolanda.
Answer: A
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 for Zelda and for Yolanda. Let and be the fractions of Yolanda's and Zelda's quizzes, respectively, that occurred in the second semester. Their yearly-average difference is Because this difference is less than so it cannot be
The other options really can occur. Taking makes Zelda's average higher; taking makes Yolanda's average points higher (and also verifies the last option after adding to every Zelda score); and taking 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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