



 
Aastha, Think of a line of n+m+1 people. You are going to walk down the line and select n+1 of them. Where could the last person chosen be? If this last person is in the n+1st place then you could choose your n+1 people in C(n,n) ways (you must have chosen n of the first n people);
Thus your sum counts the number of ways to choose n+1 of n+m+1 people in a line which is … ? Penny
 


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