Suppose A_1, A_2, A_3, ……, A_30 are thirty sets each having 5 elements and B_1, B_2, ……, B_n are ...

Suppose A_1, A_2, A_3, ……, A_30 are thirty sets each having 5 elements and B_1, B_2, ……, B_n are n sets each with 3 elements.
Let ⋃_i=1^30 A_i=⋃_j=1^n B_j=S and each elements of S belongs to
1. exactly 10 of the A^' s and exactly 9 of the B_j^' s. Then n is equal to
(a) 15
(b) 3
(c) 45
(d) None of these

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