ISI 2006.'URGENT'

[ahlashi]

Let A and B be two disjoint sets containning elements m and n respectively, and let C=AUB. The number of subsets S of C that contain k elements and also have the property that SVA contains i elements is? (V=INTERSECTION).
can anyone please help me out with this.
Answe is (mCi)(nCk-i)
C=combinations.
             

[duck]

Hi,

Think of it as selecting "i" elements from A ("m" elements) and remaining "k-i" elements from B("n" elements).