Let S be the set of the first 11 natural numbers. Then the number of elements in A = \B S : n(B) 2 and the product of all elements of B is even\ is .
Answer
1979
Solution
S = \1, 2, , 11\ has 5 even and 6 odd numbers.
The product of the elements of B is even B contains at least one even number.
Non-empty subsets with at least one even number = 211 - 26 = 2048 - 64 = 1984
(all subsets minus the subsets made of odd numbers only, including )
Of these, subsets with only one element are \2\, \4\, \6\, \8\, \10\, i.e. 5.
n(A) = 1984 - 5 = 1979
Answer: 1979