To use the wave equation, wave speed (m/s) = frequency (Hz) × wavelength (m), first check the units. Wavelength must be in metres, so a wavelength of 20 cm is converted to 0.2 m. Frequency must be in hertz, so 2 kHz is converted to 2000 Hz.
Example one. Sound travels in air at about 340 m/s. A note of frequency 500 Hz has a wavelength of 340 ÷ 500 = 0.68 m. A different sound with the same speed and a wavelength of 0.5 m has a frequency of 340 ÷ 0.5 = 680 Hz.
Example two uses the period. A water wave has a period of 0.1 s, so its frequency is 1 ÷ 0.1 = 10 Hz. With a wavelength of 2 m, its speed is 10 × 2 = 20 m/s. In another case, a wave with a speed of 5 m/s and a wavelength of 0.25 m has a frequency of 5 ÷ 0.25 = 20 Hz and a period of 1 ÷ 20 = 0.05 s. Distance = speed × time, so in 10 s it travels 5 × 10 = 50 m.