Waves

Two trucks heading in opposite directions each with speed $0.1 u$, approach each other. The speed of the sound is $u$. The driver of first truck sounds his horn of frequency 495 Hz . Let $v_1$ and $v_2$ are the frequencies heard by the driver of second truck, when the trucks approach each other and when the trucks have passed each other. The magnitude of $v_1-v_2$ is