Use the same method for every wave calculation. Write down the equation, substitute the values with their units, then calculate. The main equation is wave speed = frequency × wavelength. Rearranged, wavelength = speed ÷ frequency and frequency = speed ÷ wavelength.
Always convert to standard units first. Frequency must be in hertz and wavelength in metres. To change kHz to Hz multiply by 1000, and to change cm to m divide by 100. For example, a 3 kHz note is 3000 Hz. A sound of frequency 440 Hz travelling at 330 m/s has a wavelength of 330 ÷ 440 = 0.75 m. Ripples with frequency 5 Hz and wavelength 0.03 m have a speed of 5 × 0.03 = 0.15 m/s. A wave with a speed of 60 m/s and a frequency of 50 Hz has a wavelength of 1.2 m. A 5 Hz wave with a wavelength of 4 m has a speed of 20 m/s, and a 7 cm wavelength is 0.07 m.
Two more relationships help. The period is 1 ÷ frequency, so a wave of frequency 5 Hz has a period of 0.2 s, and a wave of frequency 10 Hz has a period of 0.1 s. A wave also travels a distance = speed × time, so at 0.15 m/s it covers 1.5 m in 10 s.