JEE Main 9 Apr 2019 Paper 2 Solved Paper

Two cars A and B are moving away from each other in opposite directions. Both the cars are moving with a speed of 20 ms−1 with respect to the ground. If an observer in car A detects a frequency 2000 Hz of the sound coming from car B, what is the natural frequency of the sound source in car B? (speed of sound in air 340 ms−1 )