1.  Write the formula of wave speed.      2.  Which of these equations are correct:            speed = frequency ÷ wavelength           wavelength = speed ÷ frequency           frequency = speed ÷ wavelength           speed = wavelength ÷ frequency
3.   Calculate the speed of sound wave that has a wavelength of 16.5 m and a frequency of 20 Hz.      4.   Higher tier only:            Calculate the frequency of the radio wave that has a wavelength of 1500 km and travels at             300 000 000 m/s.