In a secondary school of 120 final-year students, 70 study Biology, 50 study Chemistry, and 25 study both subjects. How many students study neither Biology nor Chemistry?

Answer and explanation

Correct answer: C. 25 students

  1. Step 1: Apply principle of inclusion-exclusion: n(B ∪ C) = n(B) + n(C) - n(B ∩ C). Step 2. Substitute values: n(B ∪ C) = 70 + 50 - 25 = 95 students study at least one subject. Step 3. Subtract from total enrollment: Neither = 120 - 95 = 25 students. Answer: 25 students. Why not the others: 20 forgets to subtract overlap correctly; 30 adds overlap; 15 is an arithmetic error.

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