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Each set Xr contains 5 elements and each...

Each set `X_r` contains 5 elements and each set `Y_r` contains 2 elements and `uu_(r=1)^(20) X_r =S=uu_(r=1)^n Y_r`. If each element of S belongs to exactly 10 of the `X'_r s` and to exactly 4 of the `Y'_r s` , then n is :

A

10

B

20

C

100

D

50

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The correct Answer is:
B
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