Each set Xr contains 5 elements and each set Yr contains 2 elements an

1.	Each set Xr contains 5 elements and each set Yr contains 2 elements and ⋃Xr(for r=1 to 20) =S=⋃Yr(for r=1 to n). If each element of S belong to exactly 10 of the Xr’s and to exactly 4 of the Yr’s, then n is(A) 10 (B) 20 (C) 100 (D) 50
Answer» The correct answer is (B) Since, n(Xr) = 5, ⋃X_r(for r=1 to 20)=S, we get n(S) = 100 But each element of S belong to exactly 10 of the X_r’s So, 100/10 = 10 are the number of distinct elements in S. Also each element of S belong to exactly 4 of the Y_r’ s and each Y_r contain 2 elements. If S has n number of Y_r in it. Then 2n/4 = 10 which gives n = 20

Each set Xr contains 5 elements and each set Yr contains 2 elements and ⋃Xr(for r=1 to 20) =S=⋃Yr(for r=1 to n). If each element of S belong to exactly 10 of the Xr’s and to exactly 4 of the Yr’s, then n is(A) 10 (B) 20 (C) 100 (D) 50

Answer»

The correct answer is (B)

Since, n(Xr) = 5, ⋃X_r(for r=1 to 20)=S, we get n(S) = 100

But each element of S belong to exactly 10 of the X_r’s

So, 100/10 = 10 are the number of distinct elements in S.

Also each element of S belong to exactly 4 of the Y_r’ s and each Y_r contain 2 elements. If S has n number of Y_r in it. Then

2n/4 = 10

which gives n = 20