# subset

• December 18th 2008, 06:22 AM
qwerty321
subset

Let
S be a set with n elements.
(a) How many subsets of
S have 3 elements ?

(b)How many subsets of
S have more than 2 elements ?

thank you
• December 18th 2008, 07:25 AM
Isomorphism
Quote:

Originally Posted by qwerty321

Let
S be a set with n elements.
(a) How many subsets of
S have 3 elements ?

(b)How many subsets of
S have more than 2 elements ?

thank you

(a)Its the same as asking, in how many ways can you pick three elements out of n?

(b) The answer to how many subsets have greater than or equal to 2 elements is $2^n - 1 - n$. Can you figure out why?