a siren on top of the police car emits sound at frequency of 820 Hz. The car passed a bystander on a sidewalk and is receding at speed 142 km/h/ what is the frequency the bystander hears. assume the speed of sound in air to be 340 m/s.

Question

a siren on top of the police car emits sound at frequency of 820 Hz. The car passed a bystander on a sidewalk and is receding at speed 142 km/h/ what is the frequency the bystander hears. assume the speed of sound in air to be 340 m/s. 
142 km/h  x 1000 / 3600 = 39.44 m/s

fo=fs (v+-vo/v+-vs) 
fo= 823 hz (340/340+39.44) = 737.45 m/s

so i have the answer, i just need to know what 142 is multiplied by 1000 then divided by 3600 to equal 39.44 m/s

Henry. · Answer

Vs = 142,000m/3600s = 39.44 m/s. 3600s = 1 hour. 142km = 142,000 m.