2008 AMC 8 Problem 15

Attempt Problem 15 of the 2008 AMC 8 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 2008 AMC 8 solutions, or check the answer key.

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

15.

In Theresa’s first 88 basketball games, she scored 7,7, 4,4, 3,3, 6,6, 8,8, 3,3, 11 and 55 points. In her ninth game, she scored fewer than 1010 points and her points-per-game average for the nine games was an integer. Similarly in her tenth game, she scored fewer than 1010 points and her points-per-game average for the 1010 games was also an integer. What is the product of the number of points she scored in the ninth and tenth games?
 35 \ 35
 40 \ 40
 48 \ 48
 56 \ 56
 72 \ 72
Answer: B
Concepts:meandivisibility
Small hint:
First add Theresa’s scores from the first 88 games.
Big hint:
The 99-game total must be a multiple of 9,9, and the 1010-game total must be a multiple of 10.10.
Solution:
The sum of her first 88 scores is 37.37. Since the average of the first 99 scores is an integer, the sum of the first 99 scores is a multiple of 9.9.
Since the score is less than 10,10, the sum of the scores after 99 games is between 3737 and 47,47, and is a multiple of 9,9, making the sum 45.45. Thus, the score of the 99th game is 45−37=8.45-37=8.
The sum of Theresa’s first 99 scores is 45.45. Since the average of the first 1010 scores is an integer, the sum of the first 1010 scores is a multiple of 10.10.
Since the score is less than 10,10, the sum of the scores after 1010 games is between 4545 and 55,55, and is a multiple of 10,10, making the sum 50.50. Thus, the score of the 1010th game is 50−45=5.50-45=5.
Therefore, their product is 8⋅5=40.8\cdot 5=40.
Thus, the answer is B .
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