CEMC Fryer, 2003

Algebra Difficulty 2.0 Junior Prove it Canada

Later in the year, Mary realizes that she needs a mark of $100$ on the next test in order to achieve an average of $90$ for all her tests. However, if she gets a mark of $70$ on the next test, her average will be $87$ . After she writes the next test, how many tests will she have written?

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