IMO Shortlist 1987 problem 16

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Let p_n(k) be the number of permutations of the set \{1,2,3,\ldots,n\} which have exactly k fixed points. Prove that \sum_{k=0}^nk p_n(k)=n!.(IMO Problem 1)

Original formulation

Let S be a set of n elements. We denote the number of all permutations of S that have exactly k fixed points by p_n(k). Prove:

(a) \sum_{k=0}^{n} kp_n(k)=n! \ ;

(b) \sum_{k=0}^{n} (k-1)^2 p_n(k) =n!

Proposed by Germany, FR
Source: Međunarodna matematička olimpijada, shortlist 1987