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investor.rb

class Investor

  def initialize(*args)
    @c, @f, @x = args
    @income = 2.0
    @seconds = 0
  end

  def solve
    return proper_seconds if (@x-@c)/@income < @x/(@income + @f)
    @seconds += @c/@income
    @income += @f
    solve
  end

  def proper_seconds
    (@seconds + @x/@income).round(7)
  end

end

values = [
  [30.0, 1.0, 2.0],
  [30.0, 2.0, 100.0],
  [30.5, 3.14159,1999.1999],
  [500.0, 4.0, 2000.0]      
]

values.each { |settings| puts Investor.new(*settings).solve }