S(n,2) = 2^(n-1) - 1 http://www.careerbless.com/aptitude/qa/permutations_combinations_imp7.php Follow this link, to know it in detail.
Doubt in a similar question.
How many ways are there to distribute five distinguishable objects into three indistinguishable boxes?
Can we solve it like following?
First, we calculate the no of ways of distributing 5 distinct objects in 3 distinct boxes which are = $3^{5}$ and then we divide it with 3! as we can label the boxes in 3! ways.
Final answer=$3^{5}$ /3!
I know that using the formula of Stirling numbers of the second kind, it can be solved as S(5,1)+S(5,2)+S(5,3). But can the method stated above be used?