Aspire Faculty ID #16436 · Topic: NIMCET 2009 · Just now
NIMCET 2009

$A_1, A_2, A_3, A_4$ are subsets of $U$ (75 elements). Each $A_i$ has 28 elements. Any two intersect in 12 elements. Any three intersect in 5 elements. All four intersect in 1 element. Find the number of elements belonging to none of the four subsets.

Solution

Use Inclusion–Exclusion: 
 $|A_1 \cup A_2 \cup A_3 \cup A_4|$ $= \sum |A_i| - \sum |A_i \cap A_j| + \sum |A_i \cap A_j \cap A_k| - |A_1 \cap A_2 \cap A_3 \cap A_4|$ 
 Substitute values: 
 $\sum |A_i| = 4 \cdot 28 = 112$ 
 $\sum |A_i \cap A_j| = \binom{4}{2} \cdot 12 = 6 \cdot 12 = 72$ 
 $\sum |A_i \cap A_j \cap A_k| = \binom{4}{3} \cdot 5 = 4 \cdot 5 = 20$ 
 Intersection of four = 1 
 So: $|A_1 \cup A_2 \cup A_3 \cup A_4| = 112 - 72 + 20 - 1 = 59$ 
 Elements belonging to none: $75 - 59 = 16$

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