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Of the 100 grade-12 students at Vennville High School, 85 have signed up for biology, 55 for chemistry, and 82 for physics. The science students include 42 who are taking both biology and chemistry, 39 who are taking chemistry and physics, and 70 who are taking biology and physics, and 28 who are taking all 3 science courses. How many students are not taking any of these three science courses?