A bus driving along at 39.6 kmph is approaching a person who is standing at the bus stop, while horning repeatedly at an interval of 30 seconds. If the speed of sound is 330 ms^-1, at what interval will the person hear the horn ?

As driver is honking moving towards a person then waves released due to honking will travel the distance present initially between bus and person. After 30 seconds the distance between person and bus decreases so they spread quickly so frequency increases hence the person hear the horn in less than 30 seconds.