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Hi,
In how many different ways can a chairperson and assistant chairperson be
 selected if there are seven scientists available?
Order of Selections Important.  This is a Permutation.
Number of permutations of a set of n objects taken r at a time is given by:
nPr = (n!)/(n - r)!.  
  7P2 = 7!/5! = 42 ways