John takes a series of 5 tests in his math class. His scores on the first four tests are 78, 82, 89, and 90. If each test has a maximum possible score of 100, what is the minimum score John needs on his fifth test to achieve an overall average of 85?

Using the average formula, the total required for all five tests is 5 times 85, which equals 425. The sum of the first four scores is 78 + 82 + 89 + 90 = 339. Subtracting this sum from the required total gives 425 - 339 = 86, which is the score needed on the fifth test.

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