2017 AMC 10B Problem 25

Below is the professionally curated solution for Problem 25 of the 2017 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2017 AMC 10B solutions, or check the answer key.

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Concepts:divisibilitymeanbounding to limit cases

Difficulty rating: 2300

25.

Last year Isabella took 77 math tests and received 77 different scores, each an integer between 9191 and 100,100, inclusive. After each test she noticed that the average of her test scores was an integer. Her score on the seventh test was 95.95. What was her score on the sixth test?

9292

9494

9696

9898

100100

Solution:

Let SS be the sum of all seven scores. Since all seven averages were integers, SS is divisible by 77. Also the seven distinct scores are between 9191 and 100100, so 91+92++97S94+95++10091+92+\cdots+97\le S\le 94+95+\cdots+100.

Thus 658S679658\le S\le 679, and the possible multiples of 77 are 658,665,672,679658,665,672,679. Since the seventh score is 9595, the first six scores sum to S95S-95, which must be divisible by 66. This forces S=665S=665.

The first six scores sum to 570570. The first five-score average was also an integer, so the sum of the first five scores is divisible by 55. Therefore the sixth score is divisible by 55. Since the seventh score is already 9595 and all scores are distinct, the sixth score is 100100. Thus, E is the correct answer.

Problem 25 in Other Years