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number theory senior
Problem
Last year, Isabella took 7 math tests and received 7 different scores, each an integer between 91 and 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. What was her score on the sixth test?
(A)
(B)
(C)
(D)
(E)
Solution
First, remove all the 90s, since they make no impact. So, we have numbers from to . Then, is the 7th number. Let the sum of the first numbers be . Then, and . We easily solve this as . Clearly, the sum of the first numbers must be . In order for the first numbers to sum to a multiple of , the sixth number must also be a multiple of , since is a multiple of . Thus, the only option is the sixth number is , which gives
Final answer
E