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Let "C" represents the number of correct answers.
Then the number of incorrect answers is (30-C).
The inequality is
5C - 2*(30-C) <= 66.
5C - 60 + 2C <= 66
7C <= 66 + 60 = 126 ====> C <=
= 18.
Answer. The number of correct answers is not more than 18.
Or, equivalently, The number of correct answers is at most 18.
Solved.