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Let S={1,2,,180}S = \{1, 2, \ldots, 180\}. Define AA as the set of all multiples of 4 in SS, BB as the set of all multiples of 6 in SS, and CC as the set of all multiples of 9 in SS. The number of elements in SS that belong to exactly one of A,B,CA, B, C is

Solution

Correct Option: 1

Counts in S={1,2,,180}S = \{1, 2, \ldots, 180\}:

A|A| (multiples of 44) =1804=45= \dfrac{180}{4} = 45

B|B| (multiples of 66) =1806=30= \dfrac{180}{6} = 30

C|C| (multiples of 99) =1809=20= \dfrac{180}{9} = 20


Pairwise intersections use LCMs:

AB|A \cap B| (multiples of 1212) =18012=15= \dfrac{180}{12} = 15

AC|A \cap C| (multiples of 3636) =18036=5= \dfrac{180}{36} = 5

BC|B \cap C| (multiples of 1818) =18018=10= \dfrac{180}{18} = 10

ABC|A \cap B \cap C| (multiples of lcm(4,6,9)=36\text{lcm}(4,6,9) = 36) =5= 5


For exactly one set, use:

Exactly one =A+B+C2(AB+AC+BC)+3ABC= |A| + |B| + |C| - 2(|A \cap B| + |A \cap C| + |B \cap C|) + 3|A \cap B \cap C|


Substitute:

=45+30+202(15+5+10)+3(5)= 45 + 30 + 20 - 2(15 + 5 + 10) + 3(5)

=9560+15=50= 95 - 60 + 15 = 50

Answer =50= 50

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