A manufacturer of a certain type of acoustic transmitter knows that the frequency of the signal produced by the transmitter is normally distributed wi

A manufacturer of a certain type of acoustic transmitter knows that the frequency of the signal produced by the transmitter is normally distributed with mean µ = 5000 kHz. To ensure that the transmitters produced meet the industrial regulations the manufacturer wishes to control the frequency in such a way that only 10% of all transmitters have frequency below 4700 kHz. What value of standard deviation ? provides such control?