class  Mycompute
  def  self.sum(a)
    a.inject(0){|s,i|s+i}
  end
  def  self.mean(a)
    a.inject(0){|s,i|s+i}/(a.size.to_f)
  end
end
Mycompute.sum([1,2,3,4])
=> 10
Mycompute.mean([1,2,3,4])
=> 2.5

how  can i  get  Mycompute.sum([1,2,3,4]).mean=2.5?
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