Physics Doppler effect help?

Modern International Maritime Organization (IMO) regulations specify that ships longer than 200 m must have a ship's whistle whose frequency is between 70 and 196 Hz. Suppose that a 235-m ship has a whistle whose frequency is 190 Hz. If the ship is stationary, what is the maximum speed that an approaching observer can have so that the ship's whistle still satisfies IMO regulations?