def PresentValue(future_amount, interest_rate, periods):
  """Returns the present value of some future value.

  interest rate: as a value (ie, 0.02 is the value for 2% per period)
  periods: total number of interest payments which will be made

  For more, see https://en.wikipedia.org/wiki/Present_value
  """
  return future_amount / (1.0 + interest_rate)**periods