In a class, 32 students are boys. Each boy knows five girls and each girl knows eight boys. What is the smallest number of girls possible in the class?
Note: "Knows" means they know each other.
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Each group of 8 boys "knows" the same 5 girls in minimum. Hence 3 2 boys "knows" 8 3 2 × 5 = 2 0 girls, which gives the required minimum.