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Suppose A1, A2, ..., A30 are thirty sets each having 5 elements and B1, B2, ..., Bn are n sets each with 3 elements, let ∪Ai(for i=1 to 30)=∪Bj(for j=1 to n)=S and each element of S belongs to exactly 10 of the Ai’s and exactly 9 of the Bj’S. then n is equal to

(A) 15 

(B) 3 

(C) 45

(D) 35

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Answer is (C)

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