parameterized-3.rb

def weight_avg_gross_profit(reports)
  weight_avg(reports, 'gross_profit')
end

def weight_avg_cogs(reports)
  weight_avg(reports, 'cogs')
end

def weight_avg(reports, value)
  total_weight =   reports.reduce(0) { |sum, r| sum + r.sale_revenue }
  sum_prod = reports.reduce(0) do |sum, res|
    sum + (res.send(value) * res.sale_revenue)
  end

  sum_prod / total_weight
end