Q36.

If the number of subsets with 4 elements of a set A is equal to the number of subsets with 5 elements of the set, then the number of subsets with 3 elementsof this set is:
(A) 64 (B) 84 (C) 128 (D) none of the above

Question

Q36.
 
If the number of subsets with 4 elements of a set A is equal to the number of subsets with 5 elements of the set, then the number of subsets with 3 elementsof this set is:
(A) 64 (B) 84 (C) 128 (D) none of the above

anonymous · Answer

what do u mean of number of subset ?
is it the order ??

perl · Answer

if n is the number of elements in set A, then you are given that
nC4 = n C 5 . then asked to find n C 3

perl · Answer

now we know for example (by looking at pascals triangle) 
3 C 1 =  3 C2 

5 C 2 = 5 C 3

7C 3 = 7 C 4

9 C 4 = 9 C 5 

So n = 9.  Now  9 C 3 =  84
so the answer is B

perl · Answer

|dw:1402161121560:dw|